UA's can self repair and they obviously uses the same mechanism as the Meta-alloys use to grow (same sparkles and coloured gas emission).
If you can self repair and grow only with access to metals ,you may also be able to replicate shapes. These mysterious ships may acctually be the aliens, not alien ships.
Well, that could be a very clever move from FD to avoid building new assets for alien ships! They could just use the humans ones pretending them being 'converted' to aliens!!!!
Ahaha!