Yes - this can all be done before the 'migration'. Actually - this is done in place of the migration since you won't be moving the old DR server.
Yes - you have to create the logins and agent jobs, just remember to create the logins with the same SID as created on the primary node (for SQL accounts) and the restored/replicated databases will synch with the logins, the same as when you first created the DR secondary.
The listener is setup for the AG - doesn't matter how many secondary instances have been added to the AG. Once you have added the new node you should be able to add that node to the listener if it isn't already assigned.
All you are doing here is adding a new secondary to the system...once it has been verified you remove the old secondary and the 'migration' is complete.