Yes. Providing you are not bringing the firearm in to them for shortening. The burden of responsibility is on you. Anyone can get a bbl turned down, rethreaded and install on a firearm. PROVIDING you stay with in the LEGAL overall size for your gun.
Example, you can turn a pistol in to a long gun, then return it to pistol config, BUT NO rifle in to pistol that is a SBR. The expenses incurred to illegally assemble a SBR or other NFA item is not worth the hassle or fed vs wait time for your tax stamp.
http://www.atf.gov/regulations-rulin...ing-2011-4.pdf