I'd probably just wait until you're at the other wifi site, and set it up then.
Until then, you can store the passwords safely by adding a secure note to the keychain, and putting the passwords there. Or you can use Notes on the Mac and then lock the note containing the passwords. You should then be able to copy and paste them later, albeit after entering the password for the locked note.
In Notes on your Mac, lock notes using a password or Touch ID, and unlock notes you don’t need to protect anymore.
support.apple.com
If you've never joined the desired wifi network before, then there won't be any of the necessary identifying info for that network in the keychain. Things like SSID or BSSID are stored with the password, and if those are missing, then storing just the password won't help.
There are also access-controls stored with the keychain entry, such as which process can access the entry, and those would also need to be correct.
If you have joined the network before, and there are already entries for it stored in the keychain, then you can search the keychain for an SSID and see the entry (or entries). You should then be able to edit the password (after entering the account password) and save it.
I would still keep a fallback copy of the password(s) in a locked note, in case the edited entry failed for some reason. The keychain can have a mind of its own, and sometimes the most effective way of dealing with failing entries is to delete them, then start over in whatever normal procedure typically prompts for a password.