It is currently possible to do this indirectly using Excel, mail-merge, and the temporary password feature. But a direct feature for it would be nice.

Also ability to only nullify passwords on insecure old password schemes (likely from imported accounts). Ability to select which legacy password schemes to do this for.
Mention this in the tut_importer tutorial.

A future extension would be older password_hash schemes within PHP, if bcrypt becomes insecure.

If a password has been 'nullified', a user should be told so when they try to sign in - so they understand why they need to do a reset on it.

Also it's been requested to be able to change an existing single user's password to temporary. A 'temporary' checkbox when changing another member's password would be useful.