You will have to do an addition.

The short answer is that while it‘s possible it might be advisable to just add an electric under floor matt to such a small area.

IMPO

SR

You should ask a general contractor. I suspect it could be problematic trying to run pipes, at least the sewer pipe, down through an existing floor that has hydronic lines in it, if the thought is to wall off existing interior space rather than bumping out a small addition. Also, running the water pipes and the vertical pipe to vent the drainage line might also be a challenge.

Contractors usually say "anything can be done", but that's always a question of cost and how much trouble the desired work is worth going through to the owner.