An acoustic guitar that can be plugged in is an electric guitar (acoustic/electric). Plugging it into your Mic input would probably actually work, but may not be good for your amp or other components and speakers.
When I first started playing guitar, I plugged it into the mic input of an old-school 8 track player and never had any problems for like 3 years. So you can do it, just don't be surprised if something gets fried if you push it too hard.
By the way, a small guitar practice amp will run you $29.99 at most any music store or online (Sam Ash, zZounds, Musician's Friend). My current pratice amp cost me $59.99 on sale at Sam Ash (Marshall MG-10CD) and sounds good enough and gets loud enough for me to use at small venues like coffee shops. Check it out:
http://www.samash.com/catalog/showitem. ... GroupCode=
That a small price to pay to keep from frying all your other equipment.