Help! - Turning part of an Outfit into an Accessory

Hey! First you need to move your cape uv map to different place on uv map so it will not overlap with clothes textures. Check the uv map to place your map correctly. To keep not bad quality of your texture probably hat uv map would be best, but you can't wear hats at the same time - you'll have your cape texture on hats. Then in any 2d editing program (photoshop/gimp/etc) you need to move texture of your cape to place you moved your uv map (export uv map from blender and open it in program, append cape texture and move, resize, do what needed to suit uv map).
Later you need to create package based on an accessory - could be hat one. (If you use top/outfit as a base your cape won't be an accessory, just top/outfit) If you use hat as base you need to open "Regon Map" in warehouse tab, delete one hat setting and change second layer to 0 and region type to base.
It's not the easiest thing if you never before create any cc, but you can try