User Place Holder

Yes, Quickbase has placeholders built in, though they're a little hidden.

If you import a value into a User field that doesn't match a user, Quickbase keeps it as a placeholder. Import something like TBD Inspector and it shows up in the user field with double quotes around it.

When the real person is ready, go to Users, click the arrow on the Share app with new user button and choose Replace Placeholders. Pick the matching user for each placeholder and the records switch over to that person.

Use names for the placeholders, not emails. If you import an email that belongs to a registered Quickbase user, it matches that person right away.

If you need placeholders long term, like positions that change hands, I'd add a People or Positions table with a User field and relate your records to that. You can assign work to a position before anyone fills it, then just set the user when someone does.