Sorry for the cryptic subject. I have just discovered that you can attach a windows as a tab to another window - like, if you want to mostly use Firefox for browsing, but sometimes Konqueror (as I have to, since some websites take an unbelievable time to load in Firefox, but are fast in Konqueror). Perhaps a silly example, but the feature is great.

So, the question is: is it possible to bundle applications together, so that they start up as tabs in the same frame, using this feature?

moved to Kwin forum

"Not really", but you can setup window rules for FF & konqueror to be in the same tabgroup.
If you activate autogrouping, they'll always start in the same tabgroup.