drag a roster item from one account, and drop in another another account should work
I drag foo who I have in jabber.moon acct to jabber.earth. I drop him either in jabber.earth acct row (which should put him in General atm, and in the future, put him without group) or in another group of jabber.earth and Gajim should automatically use the name I use for this account and ask for subscription etc..
user atm has to see JID, go to add contact, type JID.. when he can just move the contact :)
drag foo to another account should by default + (COPY), if CTRL is pressed that should become without + (MOVE) and in this case we also remove auth from him in the source account