You can't, and not necessary.

Reviews from users without write permission won't affect the merging status of PRs. Their approvals looks like:

This is an approval from a user with write permission looks like:

And if you don't turn on Code review limits in settings page, a user explicitly granted access can voluntarily review PRs. So just ping them. If they are willing, they can add themselves to the reviewer list.

If you still want to do that, in organization-owned repositories, you can do that by grantting them read permission. Still adds to collaborators, but permissions haven't changed. Then they will be included in the candidate reviewer list.