If they can be traded, just put your generals in the same stack and drag the ancillary or relic over from one general to the unit card of the other.

If they cannot be traded, you'll get some speech like "not possible, Sire".

It seems most ancillaries cannot be traded (tutors can) but at least some relics can. It would be good if people could post any ancillaries that can be traded and any relics that cannot.