You can definitely just steal the character generator heads and copy paste them on top of the bodies to get the hair. It pretty quick and clean as far as things go. As for making clothes it a good bit more complicated. I tried making battle sprites a while back to ok success.
Best I can recommend is to look at similar clothing using those sprites and emulate them. I would usually look at a reference image of the outfit I was trying to make and sort of outline the outfit on a separate layer on top of the template. After that I would start filling in colors, highlights and shadows as best I could, either copying the base template, another outfit that I was trying to emulate or just whatever I thought would look good. Unfortunately, as far as I am aware you basically just have to do it all manually, there isn’t really anything you can do to automate making the outfits.
Here are a few of the outfits I made when I was trying to learn spriting.