If the barrel extension brings the overall barrel length to 16" and is "permanently attached" per NFA guidelines, then you can add a stock to it. As long as the overall length of the firearm is greater than 26" then you have a Title 1 rifle. No worries about running afoul of NFA rules.
However, you HAVE to have a barrel of 16" or more. A barrel extension, to be considered part of the barrel, must be "permanently attached" and there are 3 ways of accomplishing that outlined by the ATF.
These PDF's may be of help to you: