I'm surprised someone hasn't chimed in already.
I haven't made hybrid wood/aluminium but I have done a topmast extension with aluminium/aluminium and the principle should be the same.
I don't think fastenings are desirable or necessary.
There are three forces at play which need to be considered.
(1) Bending. The standard rule of thumb is to bury 10% of the extension (same as the mast itself - the "10% above partners" rule). If the extension is 5' (say, 2m) then the "bury" or overlap needs to be 20cm minimum. This seems ridiculously small, so why not just make it arbitrarily, say, 30 or 40cm.
(2) The downward force due to halyard and lifts - the weight of the rig. The obvious way is to have a shoulder at the join, which not only creates a smooth transition but also makes it impossible for the topmast extension to "telescope" into the mast.
(3) The rotational force that the rig transmits to the mast. This is considerable, and is the only reason why you might want to put mechanical fastenings between the mast and the extension. However, the wooden topmast is presumably going to be made to fit nicely into the mast tube, and the faying surfaces will be quite great - glue should be sufficient to prevent the topmast twisting or rotating inside the mast tube. I have used epoxy glue, but I am advised that a different sort of adhesive would be better - especially when gluing two materials which have quite different characteristics. A polyurethane rubber, I believe, is a better proposition - there are various brands, for example "Simsons". The extension should fit into the mast tube with enough tolerance to allow a good body of this synthetic rubber glue.
Can the wooden topmast be hollow? Why not?
Also you could (and should) give it some taper.
You won't find a better description than Annie's, in her blog here
As an afterthought: you will note in Annie's blog, she refers to an issue of getting good alignment - the glue is soft, and you don't want to have a slight bend at the join - it might be visible. The two parts need to be colinear while the glue sets off. You want to decide how you are going to ensure this before you have applied the glue.