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.