Board Thread:ROBLOX help and discussion/@comment-26968432-20160311001831/@comment-26374715-20160311183446

As said, you can't officially make them, but you can in Studio. This is called a retexture, and non-admins can get a retexture published, making ROBLOX publish it as an official item.

There's a group that Brighteyes is in that explains a bit about it.