This would be rather hard to do. Here is the cog diagram for suntour freewheels (I snagged this from an ebay auction a few years ago) :
http://www.ece.ubc.ca/
Regular 5-speed uses ETRAA cogs. Ultra 6-speed uses STTRAA cogs.
So you would throw away the 'E' cog, and then add 'S' and 'T' cogs. However, what's worse, you need special 'U' spacers to replace your 'B' style spacers. So, while you need 2 cogs, you need 6 new spacers, and so conversion from 5 to 6 speeds is very spacer-intensive on suntour freewheels. I think Suntour is the only company to use the same body for 2 or more spacings (5,6,ultra-6, ultra-7). I think there were several companies who did regular-5 and regular-6 by simply adding a special small cog with additional threads for a 6-th cog (i've seen this on Atom and 1st-generation Dura Ace freewheels). But generally, the compact-freewheels (e.g. Atom 77 = compact) used entirely new bodies, I believe.
- Don Gillies
San Diego, CA, USA