My belief is that, since Data Protector Express is a totally different product, and not sharing any components or Services with Data Protector that I am aware of, you can probably install DP on the same server as DPX
Now, having said that, no one that I know of has ever tried this, so I can't be absolutely definitive (maybe one of the other folks on the Forum has) that thsi will work. Prepare yourself to Fail by doing whatever you need to do to save your current DPX configuration and Database