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if you have the screwin filterholder, I believe any 52mm threaded filter will work on this. The challenge is that you can't adjust the ND settings without pulling the filter holder out of the lens, and then you're just inviting dust and other cr$p into the lens. I would recommend getting fixed ND filters as Jay has mentioned, and developing a sense for when to use which one.
The other drawback to the variable ND filters is that if you really crank them up, you get an "X" pattern in the image with the outsides being darker, and it happens even with the good filters (I bought a Heliopan variable ND this summer and had this issue). Effectively, the variable ND is two polarizers stacked, which I understand causes this issue as the ND value increases.
I think the cost of two ND filters will also be significantly less than the variable NDs.
just my $0.02,