When I created a new account some time ago I started to add every contact I had in my previous account to the new one… takes some time…
Today I discovered that there is a way to synchronize all contacts from one account into another in Gajim :)
Edit -> Accounts -> Select the new account -> Open „Administrative Operations“ -> Synchronize contacts -> Select the previous account -> OK
Now select the contacts you want to be synchronized.
Cool :)