Sorry but it looks confusing. Do you mean joining an additional SQL Server instance as secondary replica or add a Listener resource (new IP Address, and new network name) to de cluster resource group?

In the first case, best option should be restoring full database backups and transaction logs on the candidate instance till it is synchronized to production and then joining to the AG.

In the second case you need to do that on the primary server. Test it on a pre-production environment before to see any possible issue and practice the procedure.

I hope this helps.