You can already do this, but I am not sure if and when this ability will be added to the UIs. If your inboxes receive non-chat messages, there is no way to tell in the UI which inbox the message comes from and which address you should use to send the reply, currently it will just be sent from the selected default/primary relay and in the future we may want to send automatically from one of the relays. This is likely not what you want if you use the same addresses for e.g. replying to unencrypted mails.
If you have created dedicated addresses for chatting on non-chatmail servers, made sure you do not use the same mailboxes for other email clients and for other Delta Chat profiles, and want to use such mailboxes in a multi-relay setup, you can do this with DCLOGIN QR codes: