Actually it's a combination of Fuikai and Ellias' answers.
No, you may not make (as in create for use) custom droids, ships, etc. The only exception are custom items. One such item you may have seen already are the Easter Eggs that are left over from this past Easter holiday. Custom items have no use except for roleplaying.
As Ellias said, you may have an artist create a custom image or art for any entity you have in your inventory. For a certain amount of combine points those images can be applied and will remain there until removed by the owner of the entity.