Hi there,
I just checked the Help section on my copy of RootsMagic and it confirms that you should be able to drag and drop between databases, so it looks like a restriction from a Trial version.
Did you upgrade from a Trial, or is it a genuine version? If you didn't have a trial version, RootsMagic support may supply the answer.
To check your version, click on the Help tab at the top of the screen, then click on "About RootsMagic" on the drop-down menu. This will tell you your Version number and who it is licensed to - in my experience, this normally indicates whether you are using a Trial version.
I'm on Version 3.2.6, which I think is the latest - haven't checked for updates lately.
Good luck!