Have all users be global users

We need a way to turn all our existing users into global users, and we need a way to make new users global users even if not using the MS sign-up form.

Details:

We would like all users to be global users. This is stymied by several problems.

We have preexisting users who were registered before the site became a network.

We have new users who register using the normal registration page instead of the modified sign-up page, because we much collect extra info about them via the s2Member plugin’s modified account creation process.

Some of the added fields collected during the modified registration process cannot remain editable after registration, so we cannot simply use the plain sign-up form and then bring them somewhere else to edit their profile.

We need a way to turn all our existing users into global users, and we need a way to make new users global users even if not using the MS sign-up form.

Does anyone have any advice for how we can do this?