Adding fins?

I got a pretty newb question since i am an newb to surfing.... anyway heres my delema (if thats how u spell it) i bought a really cheap board of my friend for 30 buks its pretty old, its also a short board with one fin... and everyone tells me that surfing one fin on a short board is tough and since i havent gone b4 i was wondering if you can add fins (probably 2 to make it 3 obviously...) and if you can how much would it cost. Thanks in advance.