There is no direct link in France between SEP and microlights,(any category) but as there is no minimum requirement for the microlight licence and a PPL exempts you from the written papers it isn't a big problem. Declaration of start of training to the DGAC, enough flying to satisfy your microlight instructor, test. Job done.
Once you have one category any of the others can be done by differences training and test.
Microlight to SEP is less easy. No concessions, do the 45 hour minimum, writtens, x/c and test as if you have never flown at all. But in fact most people who do that get their licence in the minimum time, so there is some practical advantage.
A UK microlight pilot coming to France would not have any problems, except maybe doing the radio licence (which is not obligatory, but useful if you want to do any touring)
The french microlight licence is for life, with a GP medical required (once) only if you wish personal accident cover through the FFPLUM. It's not required by the DGAC.