A diagram might help a bit but what I understand is that you have a mast step on
cabin top and underneath that a
compression post and the
compression post goes to the
cabin sole and atop a floor (thwartship frame) of timber or steel. What you are asking is if there should be a support spreading the load from one floor to the other?
I'd say that your compression post should extend it's load to the
keel. If there is a space between the bottom of the compression post to the top of the
keel then that space should be filled with a noncompressive material that can live in a
bilge or there should be something very strong to bridge that gap from one floor to the other to spread the load on top the keel and up a way on the
hull.
Do your blueprints or plans have that detail?
Just my opinion.
kind regards,