Yes you can accomplish the flap cycle on the way to the runway. Boeing doesn't address how to accomplish this flap cycle while taxiing on slush or snow covered taxiways. Boeing suggests delaying the final t/o flap selection on contaminated taxiways, so I would think that Boeing wouldn't recommend a cycle to Flap 40 on snow/slush/ice covered taxiways (which stands a much better chance of getting contaminants on the flaps than a normal t/o setting of Flaps 5).
Caution!
Flap selection should be delayed if extended taxi through slush or snow conditions is anticipated.