Make a layout and name it Chara_Gen. Make sure it is loaded BEFORE your main game.
Set each group of customizable items in their own sprite above your ragdoll. (Such as spr_hair for hair)
Find the window that says "Animation Frames".
Right click and choose Add Frames.
Select each different resource (brown buzzcut, blonde mohawk, etc..) and import them as Frames.
Create a button or drop down list for each Sprite.
In the event window:
Make a global variable for each custom resource (hair,beard,gloves,etc..)
Create an "on layout start" event, then set the animation speed to 0 for each sprite.
Create an event upon the appropriate gui interaction that increases or decreases the corresponding variable by 1.
Create an event "On tick" then set the Frame for each sprite to the right variable. (spr_hair would be have the frame set to var_hair)
EDIT: I'm working on this same sort of thing myself, this is the system I used. Good luck!