A propane torch with aluminum solder will work. Move the torch along the work to heat it evenly. Melt the solder by touching it to the work, the torch should be aimed where the solder is melting. The solder will puddle and flow and can be guided by moving the solder stick and torch. After you're done, the joint can be ground and sanded to a seamless, smooth finish. The muffler can then either be polished (shiny) or bead blasted (matte) as a finishing touch.

Of course, clean, clean, clean the muffler and extension before you start.