Add Receptacle Using Power Source from Doorbell Transformer?

Hey Fellow DIY'ers, I want to hide my subwoofer inside a closet in my media room. I can easily route the audio cable into there, however, I cannot do the same with the electrical power. But....in that same closet is the transformer for my doorbell (see attached picture). Would it really be an issue to pigtail the incoming power in that box and run romex down to a new electrical outlet about 3 feet below it? Attachment 38282 (http://www.doityourself.com/forum/attachments/electrical-ac-dc/38282-add-receptacle-using-power-source-doorbell-transformer-image.jpg)