You're going to need a proper rigger-engineering consult here. The dynamics of a deck stepped and a keel stepped mast usually are quite different. Keel stepped can be lighter by a little bit if the partners are correctly engineered and the mast is held well in two places - step and partners - before relying on rigging above that.
So presumably you're getting a stick - wood or aluminum? - that's a bit beefier than strictly needed and whatever its old wire may have been could also be heavier than you'll need if you go keel stepped. Think that whole bit through before you even ponder keel or deck.
I don't have metal working experience with extending an aluminum tube but it's done often enough that it should be simple enough, especially if the join is at the partners where the hard spot of a sleave or insert for the join would be placed. Certainly if the stick is wood, a nice long clothspin scarf that follows the construction of the mast (I presume hollow) down past the partners (compressing plug there) to the solid mast heel that might be a foot or so high is perfectly feasable.
If you want to go deck stepped, that should be very simple as you already have plenty of structure to land both the top and bottom of the compression post.
G'luck
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