Add basic level to normal multi site sign up

Hi,

I use pro sites. I want to use the normal WP sign up because it's simple. I also want user to just start with the basic level and afterwards upgrade. How can I add the first level to every normal sign up? If I sign up now I have to manually manage the level of the user.

An alternative would be to select a default level at the pro sites sign up page and hide all levels with css and just show the sign up form.

Let me know what you think is best.

Regards,
Maurits