*UPDATE*
Went to an irrigation supply store where I was able to get a 3/4" to 3/8" (MIPT x FIPT) PVC threaded reducer fitting. I screwed this over the existing nipple and then used another reducer fitting to get up to 1" and then simply plumbed into my existing return line. I could have done like some of you said and just slipped the clear vinyl tubing over the nipple but I wanted to hard plumb it in to my existing 1" line so an adapter fitting was necessary.
