If it was manufactured as a pistol, and has had a detachable stock installed which has been rendered non-removable, it would likely not be importable because it was originally made as a pistol.  
If it was originally manufactured with a fixed stock, then it might not be deemed to be a pistol with a stock.
This could not be easily done if US made.  Short barrel plus stock = NFA short barreled rifle, US Federal registration.  Really complex on US side.