I've mixed in some Valejjo white with my Lifecolor paints in a pinch a couple times and at first it looked like it worked OK but as I began spraying, I could see it acting funny in the color cup. Nothing crazy, just appeared to be separating.
As long as I kept stirring it occasionally, it worked out fine in the end and I had no problems with the paint on the kit. I wouldn't make a habit of it but it did work those few times I had no choice.
HTH
Ken