It would basically be a parts swap. The problem would be finding the Roadster parts at a reasonable cost. You would need the peg rails, brake pedal, shifter, shift linkage, and brake lines. I don't know if the Roadster has a different rear master cylinder or not.
Also.. I was just looking at a Roadster the other day. Yes the pegs are farther back, but Triumph has put a much taller seat on the Roadster to compensate for the leg room. I bet the Roadster is a good 2-3 inches taller than my RIII Standard.. and that's with me running a 55 series rear tire. I was surprised at that. I can easily flat foot my bike with knees bent. Sitting on the Roadster I was almost on tippy toes... I have a 32" inseam..