Making something like that is incredibly simple and I would assume if you were making a game you would already have the basic knowledge of actionscript required to program something like that.
Heres how I would make a custom character maker.
First I would make an array called customchar, then have as many values for it as customizable parts in the character. Then I would make little arrow buttons that let people adjust the array values. Then I would make it so the characters body parts' movie clips would
gotoAndStop(customchar[partnumber goes here] ) thus making the parts go to another frame with a different looking body part.
Don't know what the hell I am talking about? Then go here and learn some as http://www.newgrounds.com/portal/view/34 4206