PDF sales, different content from free?

Ok, this might be a weird question and I'll try not to be confusing. 

For a while I've had PDFs of my comics that bundle the free webcomic content I have, plus extra stories that are only included in the paid version. I'd like to add these to GlobalComix, but I'm not sure if this is possible besides creating a separate upload of each chapter for the paid PDF version with extra content, in addition to the free version.

Is there a way to do this that keeps both versions "attached"? Like, you can read the free version, and then buy the PDF with more content if you want? I'm just not sure what the best method for this is with the features that are available here. I hope this made sense!