The pack would have to store that info then...

when you paste an nprofile to add it to the pack, it would get a relay from inside the nprofile and add it as a hint to the p tag. nprofile is pubkey + relays bech32 encoded. If the user is adding an npub you could attempt to fetch their relay list from a bootstrap relay list. You then would take one of their relays and add it as a hint If the user types a nip05, you can try to get a relay from their nip05 data and use that as a hint (my nip05 has a demo of this). If there are no relays on their nip05 you could try to fallback to the method in the previous paragraph