If your variable ND filter has threads on the front, then you can attach any sort filter you wish to the front of it, including square filter holders.
It can be tricky screwing filters onto the front of ones that rotate, such as your variable ND, or polarizers, but it can be done. I sometimes do this with my standalone polarizer filter mounted onto my lens, with a Lee filter holder attached to the front of the polarizer. Adjusting the polarizer is a bit tedious, so I usually screw the Lee filter adapter onto the polarizer, compose the scene, adjust the polarization of the filter, tape the polarizer to prevent it from rotating, then snap the Lee square filter holder onto the Lee adapter in order to use my GND filters.
If your variable ND doesn't have threads, then perhaps you could bodge it together with tape. Personally, I wouldn't do this, and I don't really recommend it, but frankly, a lot of photography is done with creative "off-label" use of things.
As an aside, variable ND filters aren't really that much use in photography. They seem like a cool idea, the ability to "dial down" incoming light quickly. But if you are already using square filter holders for your GND, a small set of ND filters is about the same price as, or even cheaper than, a variable ND. Also, because variable NDs work by polarization, with wide-angle lenses they can produce strange X-shaped darkening when shooting outdoors.
