Good question. I don't see why not. Its speed may be slower because WS-20 has larger bypass ratio.
Both Y-20 and H-6 have been using D30-KP-2 engines. The WS-18 is intended as potential replacement of D30 for both aircraft. There is no reason why WS-20 can not be used on H-6. However, I think right now the Y-20 and YU-20 are prioritized for WS-20.
I think the biggest reason for not doing so is because of the structural rework that will be needed.
WS-20 has different dimensions to D-30, specifically it is wider. The location of H-6's engines being at the wing root means they'd have to spend a bit of time and resources to re-engineer it and do another cycle of flight tests to verify everything.
Given we are probably at... what, over 130+ H-6K/J/N airframes combined now, I do not see them building many more H-6K/J/N airframes, perhaps a couple dozen or more.
I don't think it'll be worth developing a whole new H-6 airframe to accommodate WS-20, given that.
If they were looking to build another 100+ H-6 airframes, that might be a different matter.