Hey @IsiGB !

Being that the free and the paid accounts are completely different instances of Canvas, there really is not a way to join the accounts.  What I would suggest doing is to export the free Canvas course as a .imscc file (from the Settings page). Then you should be able to import the course file into a course in the paid account.  

How do I export a Canvas course? (Free course)

How do I import a Canvas course export package? (Paid account)

Hope this helps!