If your 13-28 6-speed is really a cassette and not a freewheel, you could get a couple of 16 and 34 loose cogs
http://www.loosescrews.com/index.cgi...id=34154805759
http://www.loosescrews.com/index.cgi...id=34154805759
Your 13-28 is probably 13,15,17,20,24,28 so you could remove the 15 and 17 and replace with 16 and 34 to get 13,16,20,24,28,34 - or you could just get the 34 and replace the 28 with it to get 13,15,17,20,24,34 but that last jump from 24 to 34 is really big (although should work as Shimano's megarange 7-speed does the same thing with 11,13,15,18,21,24,34)
You would likely also need to use a Shimano MTB rear derailer. If you have an MTB with Shimano parts, borrow the derailer from that. Otherwise new MTB rear derailers are inexpensive.