I’ve definitely mixed burnt umber with white. However if that doesn’t work for you use another color. I would do an experiment to see if that actually happens on a scrap board.
What I loved so much about burnt umber was that it dried so fast. Therefore I mixed it in just about every part of my paintings.
I would just steer clear of using any white in the dark middle tones and darks. You can consider lighting the burnt umber via the board by applying the paint translucently.
Hope this helps,