Probably not as the Ultegra rear derailleur is unlikely to work with a cog that large. Shimano claimes that rear derailleur has a 27T limit but experience says 30 or even 32 teeth usually works but 34T almost never.

You could substitute a Shimano MTB rear derailleur and that will solve the problem. Any Shimano 9-speed MTB rear derailleur (not the newer 10-speed Dyna Sys MTB derailleurs) should work and will allow your current shifters, etc. to be used.