Depending on the sculpt you're starting with I'd removed some of the resin/sculpt in order to make a surface for your materials to stick to and then just sculpt on top of it. I did this to change a hatless Indy sculpt in to a Deckard. I used a 50/50 mix of super sculpey and sculpey firm.
I know some people prefer the 2 part apoxie's that air dry... but I find them difficult to work with.
I would recommend apply small dabs of clay to build up variations in the hair. Then sculpt the strands in. Hair doesn't grow in sheets and this is a way to help start visualizing the variety.