After the import, the interface on WG's 'create presentation' will look like Google Slides, but in fact is still in WG. You can edit the imported Google Slides. (If it is a PPT that is imported, then each imported PPT slide is a non-editable slide in WG.) You can of course add new WG slides. For the imported Google slides, if you want to edit them, you have the option of editing them within Google slides. If that button is clicked, you will be taken back to the original/imported Google slidedeck. Then if you go back to the WG presentation being edited, if you click on 'update' in the top right corner, then the changes you made in Google Slides a moment ago, will be shown here in WG. (Of course, this action is optional.)
TAKEAWAY: It may be better to import a Google slidedeck rather than a PPT since (a) each imported slide is still editable, and (b) you can also do the editing in the original Google slidedeck and update in WG.
