Whilst both M&M and Mileage Plus offer one way tickets paid for by miles, they tend to be only for the home carrier of the programme:
Mileage Plus – United (and Continental)
M&M – Lufthansa, Swiss, Austrian etc
There are ways you can covert from one programme to another programme via third parties. However, I believe the conversion rates are so poor it makes no sense to do it.
You will probably be better off buying an in-expensive one way ticket with United.