The product page on Bosch's website shows an adapter for fitting hoses to the port. Unfortunately, I didn't see a port size listed, so without an ID or OD for the port size, it's hard to know what to recommend as far as adapters go.
Bosch also uses a 27mm fitting and even sells a 27mm hose almost the same as the CT15 comes with. Might it just require a little more force to push it on?
I can also highly recommend the Bosch hose. I actually like it better than the one that came with the CT15. And it's fully compatible with the CT15 on the vacuum end. It's a little heavier but the tool end is more flexible rubber.
Bosch also sell a handful of adapters. This one has proven quite useful to me. Here's another you might find useful.