Not sure why you would want to do this, however, how high off the ground is the existing floor? is there ventilation to the existing timber bearers and joists? How big of an area are you looking at doing if doing over a large open area the weight of the concrete tile slabs may become an issue.
If just a small area IE entry you could sheet the area with tile underlay and screed and bed the slabs, that way you can even end matching, of course you will have a small step.
It will depend on what you are after as and end result, to give a more informed response.
Brian