The tried and true method for such migrations is to have the old tool play back while the new tool is recording. But that's only going to give you - say - between 10 and 50% of the conversion task (depending on the nature of your scripts).
You will be able to move over your true VB functions (Cut and paste). You can also move over some of the other code and QTPize it expecially for the window calls, button presses, etc. But that is all I can think of that will be somewhat easy.