Free site sign up with Pro Sites

How can I let my users sign up for a free site, when I am using Pro sites?

I want them to be able to start a free site. Then I want them to have the option to purchase up to 2 more levels.

But I can not set up a free site in pro sites. Are they supposed to sign up for a site using normal sign up page? I would love it to be integreted in the pricing table, using Pro Sites…