Try setting the import to "Overwrite existing entries with records matching ids". It will attempt to load existing rows with matching ids first and update them otherwise will create new entries. That might allow preserving the ids, but again this was never meant to be some sort of backup feature.

Have added 2 feature tickets to improve the user import process.

forge.joomlapolis.com/issues/9289
forge.joomlapolis.com/issues/9290

Will take a quick look as might be able to have both implement today if they're simple enough additions.

It does not appear possible to create Joomla users with specific user ids through API. Joomla simply doesn't allow for this use case resulting in the Joomla user failing to be created. Passwords will be possible though by allowing support for an "existing_password" column that bypasses encryption. So you won't be able to keep the user ids, but you'll be able to safely import already encrypted passwords.

There's unfortunately nothing I can do about that. Your only option is to directly import users through your database. Joomla user creation process simply will not allow the preservation of a specific id for new user creation. It's possible for CB itself to support this, but the user won't exist in Joomla and thus no real point in implementing. We'd have to completely bypass Joomla API and implement our own means of creating a Joomla user and that's a lot of CMS specific code we really don't want to have to maintain, sorry.