First off, welcome to the forum. Second, I regret to inform you that we have not yet found a way to do it, but we're getting there.

The secret probably lies in custom firmware (decryption plugins and the like), WipEout Pulse's SDK (which was graciously and/or carelessly left in the files for the PS2 version), and some clever texture editing tools (such as Paint.NET)

As for a way to download existing custom skins, the only way that I know of is to find somebody who already has them and transfer the savedata. I haven't had much luck in that field.