Welcome Jose, I think your question is a very good one. Annie is perhaps too modest to mention that she has successfully made timber topmast extensions to aluminium tubes on two different boats, and this may well have inspired others to do the same.
Upside down or right way up, I can't see that the equations would be any different, but the numbers would be different, because at the lower end of the mast, bending forces will be considerably greater than at the top end. So if there is any weakness in principle, in the idea of a hybrid mast, then the join detail will be much more crucial in the case of a bottom timber extension.
The question interests me, (a) because I am considering doing the same, and for the same reasons of economy, on a current project of mine. (We don't have access to aluminium lamp posts in New Zealand, unfortunately). And (b) because back in 2017 I posted the same question, in almost identical terms: "While on the subject of composite masts, I have noted with interest the composite masts made from stock aluminium tubing, with the top part made from wood - and wondered if anyone has thought of doing it the other way round - that is, making the base mast with solid timber and the top part with much lighter aluminium." I received advice from a member whose knowledge and experience I quickly came to respect, as follows: "...and it does seem that making a wooden lower portion to mate with the aluminium at the point where a PJR-sized mast has decreased in diameter to ... would be one way to go..." I left out a crucial number here, because the context of the discussion was a smaller mast for a smaller boat than your 55' Samson, but I think we can infer that in principle you can do it, provided the specifications of each of the two parts of the hybrid mast are within the envelopes recommended by PJR, for the respective material and the respective height above partners. (PJR = Practical Junk Rig by Hasler and McLeod). In practice I think you need to provide the dimensions/specifications you have in mind, and have them approved by someone with the appropriate engineering knowledge (and/or direct experience).
You can also find and go back through all of that 2017 thread. (Go to the "Forums" menu to the left of here, click on Technical Forums and in the search box enter the name of the thread, which is: "Mast materials and Specifications"). If you want to scroll through it, the exchange referred to above was dated 24 Apr 2017 04:00. If you read further, you will see there is also the issue of what type of adhesive to use, between the two mechanically dissimilar materials - and a question of how to cure a polyurethane rubber glue which is perhaps preferable to the more rigid epoxy. Your question of timber shrinkage raises further questions regarding the desirability of sealing the timber completely with epoxy first - there was a difference of opinion on that point and I will venture to add my own, which is that if timber shrinkage is thought to be a problem, then you should season the timber as much as possible and seal it up with epoxy prior to assembly, as Annie did successfully on at least one of hers. Some kind of adheisive will be necessary, by the way, because a junk rig puts rotational forces on the top mast and one would not want the top part to rotate at the join. Some kind of glue ought to be enough, however, personally I wouldn't like to use pins, screws or rivets anywhere on a free-standing aluminium mast section. As for bending forces - presumably the rule of 10% bury applies, but I do wonder if there is a risk of the circumference of the aluminium tube stretching and deforming at the point where where the timber plug exits, and therefore as a lay person, I see no harm in doubling that amount of bury (at the very least). There should be a step increase in diameter of the timber plug where it exits the tube, in order to fair or smooth the transition between the two parts - this will also take care of compression forces from halyard etc which would otherwise tend to telescope the timber plug further into the tube.
Following that exchange in 2017, I went on and made my mast from two pieces of aluminium tube of different diameters, thus I have not actually made a timber extension myself - and as far as I know, no-one has reported doing it with solid timber as the lower part of the mast. So you may be a first. Considering the distribution of weight in a hybrid mast made from solid timber at the base and aluminium tube above, it seems to me to make a lot of sense, in principle. I guess it all comes down to the scantlings you are talking about. If only a short extension is needed, you might also consider making a lower mast extension in the form of a keel-stepped tabernacle and thereby achieve an additional advantage (though nothing would beat the economy of a nice fir log if it's free! That's something else a bit hard to come by here in New Zealand).
(You know, one of the first tradeable resources that attracted early European mariners to the shores of New Zealand was the ready availability of tall straight timber suitable for spar material. (At this time supplies were running short for the British Navy). Sigh... You can't even purchase a nice straight piece for good money now, let alone find it washed up on a beach!)