is it possible to take command of abandon citadels, if yes… how does one do this??
No, that is not possible. All you can do is anchor your own structure, declare war against the owner of the abandoned structure, then go blow up the structure and loot it.
If it’s in Nullsec or WH space, you don’t even need a war dec.
… then you had better not dilly dally. It might not be there for very long.