If a sail is listed as Mylar/Kevlar is it a composite of both
materials?
Yes. Usually Dacron with a mylar sheet laminated to it for the panels.
Kevlar tapes are sewn along the stress areas.

Often the scrim at the base of the material is the Kevlar, Spectra, etc.
The Mylar is usually the film that is laminated either side of the scrim.
Often (On sails made for other purposes than racing) there is a taffeta
layer applied to one or both sides of this sail, producing an up-to 5-layer
sail more often used for cruising, ('Cruising laminate' sail) but able to be
handles with less care than a pure racing sail. (3 layers = 'racing
laminate' sail) Pentex is actually Dacron scrim with the Mylar laminate.

The tapes aren't always Kevlar and are often glued, these days.

Can a sail of that type be converted from hank on to roller furling?
I don't see why not. Mylar/Kevlar sails are made with luff tapes to fit
in a foil on the headstay. Why would you bother, though? A Mylar/Kevlar
sail is cut for performance with a roach for the prevailing wind
conditions. Roller sails have to be cut much flatter.

Yes. My older genoa was once a roller-reefing sail, but after a small
accident with the furling gear I had it converted to hanked on. Conversion
cost me about £80.00 at a local sailmaker. I must agree about the cut being
different, though.

Is it a cost effective conversion?
Probably not. Why ruin a good sail?

Why not make a good sail available for use when it previously couldn't be
used?

A