Interesting approach! I wonder whether any realization of your idea already exists...
So, basically, you are talking about using a classic x-ray source (or even EUV?) along with some cristal which is supposed to allow altering of the original wavelength. The only problem here is efficiency...
Such a device would be a major breakthrough for both research and practical usage. This is not my area of expertise; therefore I hope to get some help here.
AFAIK there are four possible directions:
1. Gas discharge. It seems not to be tunable and could be applied for EUV only.
2. Beta...