If you're careful you can take a hole saw (used on a drill) and cut a hole about 1/8" larger than the outside diameter of the pipe. Gently insert the pipe from the inside wall through to the outside wall (this will prevent the drywall from breaking outwards. Once the pipe is in place, put a thin bead of caulk around the pipe and smooth it with your finger. For a finer line on the pipe, wrap it with a piece of scotch tape and then take the tape off after you've smoothed off the caulk. Once your're all done, retape the pipe again, paint the caulk and wall, and then remove the tape so you have a clean paint line.
BTW, be sure to caulk the other side of the wall as well ... two attachment points will help minimize the chance that the pipe will shift after it's in the wall and thus damage the interior caulk / paint job.