Is it possible to create a frame within a form for them to sign up for an account with a 3rd party provider?

We have a multi-step form, and on one of the forms we'd like to send them to an external page to sign up for an account, but within the Jotform so we don't lose them. It's Step 3 of a 4-step process, and we  need to keep them on our form to complete the process. We can't do it at the end with the Thank You page because they need to go somewhere else at that point.

Yes, you can add an external page or another form into your form.  This can be done by using an iFrame widget:

http://widgets.jotform.com/widget/iframe_embed

This will allow you to load a page within the form in its own frame.