How do I build A texture from decals?

My current project is a car racing game that has a focus on customization. Part of this customization is decals. To make the game run better I would like the decals to all be compressed into one texture that is then applied to the car when I view it but when I go to edit the decals again the layers will be saved and then when I exit it is compressed into one texture again that replaces the previous one. Does anybody know how to do this or have any links that point me in the correct direction. Thanks, Zac.