An ATA is just that, an additional team ability (used to be alternate, but I digress)

To be assigned, the character has to meet the prerequisites

However, once assigned, it is as if the character always had that TA

A wild card can borrow a TA that a friendly character possesses (2 notes. ATAs are possessed and, unless specifically forbidden to, wild cards can copy any TA possessed, whether by the symbol on its base or because you assigned an ATA to the character)

Actually, it specifically states you CAN copy ATAs in the rulebook! The next to last sentence of the Additional Team Abilities paragraph (page 18): "These team abilities are possessed by the qualifying characters and can be used by wild cards (unless the ATA specifies otherwise)."