I already have a profile and I don’t want to lose messages in it. Can I create a profile on nine.test.run and use it as a relay for an existing profile?
Yes, that is the point of multi-transport. Just click on / scan the invite link on nine.testrun.org.
Also see FAQ: FAQ - Delta Chat
(Although, as you worded it, you don’t need to “create another profile”: you just add a relay to your existing one)