Excel97 and VBasic 6.0 Forms

I write an application based on Excel97 and VBasic 6.0. I want to mix Excel Worksheets and VB Forms to built the GUI.
Within Excel97 I can create so called UserForms within the VBasic Editor, but I cannot import Forms created with VBasic 6.0. On import I get the message "can't import forms of that type". (Within VBasic 6.0 you can import Excel97 userforms, given to a Designer subfolder in the projekt folder.)
Does someone know how to convert arbitrary VBasic 6.0 Forms so that I can simply
use it within Excel97 ? Or it is definitely impossible ... ?
Thanks for any answer ...
Dieter