Extending a hard-to-access exhaust fan duct
I don't know of a junction that would snap to an existing pipe.

However, with it being only a foot away, I think you could attach some flex and persistently stick small bits of foil tape around the joint. (Go the long way... not the circumference.) You may have to cut a few short nips in the flex to help you get it over the hard pipe.

Alternately, you could cut the ceiling, hard pipe it perfectly, then patch. (But nobody likes that... even drywallers.)