No, you cannot put rings that small on an Ultegra crank. The smallest middle ring possible would be a 38. You could possibly do something like 48-38-26 or go to a 110 BCD crankset that will accept smaller rings.
Take a look at TA Specialties Alize chainrings at the Sheldon Brown/Harris Cyclery site or Peter White's Cycles site.
Another consideration is a compatible front derailleur. Your standard Shimano triple road derailleur is pretty tolerant but for a big change in ring sizes you may need something different.

Al