For adding coat-tails, lengthening jackets or shirts, etc, I like to use epoxy ribbon, a two-part ribbon that you can find in about any auto-parts store.. It's yellow and blue, tear off a chunk and knead it until it turns green.. Then press it paper-thin between two sheets of waxed paper..
For lengthening arms and legs, and filling in torsos, I use either Testor's White (thins with rubbing alcohol, so it doesn't attck your styrene while you work it) or Bondo Red Glazing Putty, again, available at any auto parts store.. Thins with Testor's Liquid Cement...