Can I size .270 brass to 25-06 in a single step, that is simply run .270 brass into a 25-06 die? Will this cause any concentricity problems? Reason I ask is I have access to never-fired .270 brass and would like to use it for a 25-06 if not too much of a hassle.

Sure you can, but don't forget to trim it to the proper length.

You might also check the neck wall thickness after sizing and before loading or the finished OD after loading a dummy round to make sure it's not too thick.

Chances are there's no problem, but better safe than sorry.

you will notice when you resize the 270 to 25/06 at the base of the neck a donut will form this could cause some hazzards with a combination of thick wall necks.