I am sorry but you cannot change the AC Input of the 6834B. I think that it would require you to change a bunch of different assemblies and it would not have been cost effective.
We actually do that here for supporting our 400 V 3 phase input units. We have a step up transformer (a giant grey box on wheels) that we hook up to the US line to get 400 V 3 phase out. Unfortunately, it was an in house thing so I can't give you a model to buy or anything. One thing to make certain of if you go this route is to get a transformer rated for a high enough VA.
I am sorry but you cannot change the AC Input of the 6834B. I think that it would require you to change a bunch of different assemblies and it would not have been cost effective.