In my opinion, why add new parts to try to work around it? You're gonna have to wait on a parts order anyway to get a threaded-to-threadless adapter. May as well just spend that waiting time on getting the right-sized handlebar, and meanwhile you can return the wrongly-sized one and save money.

Since the 26.0mm and 25.4mm sizes are so close, I think you should confirm with a caliper. If you're very careful measuring, you can wrap a tape measure around and divide the reading by pi to find the diameter.

circumference of a circle = diameter x pi
...and therefore...
diameter = circumference / pi

That will tell you the right diameter you'll need, in theory... but with only .6mm margin of error, that'd be enough to scare me away from wanting to try to measure it by hand in that way.