Yes, sometimes—but it depends on the variable ND.

Many screw-in filters have front threads, so in principle you can stack a square filter holder onto the front of the variable ND. However, variable NDs usually rotate to adjust density, which makes this awkward. Mounting a holder on the rotating front can make adjustment tedious, and some variable NDs may not have usable front threads at all.

If your variable ND does have front threads, you can likely attach the holder, but expect setup to be fiddly. A common workaround is to set the variable ND first, then attach the holder. Some holders also accept multiple filters themselves, which may be a cleaner solution depending on your system.

If your variable ND has no front thread, then using that screw-in filter together with a holder-based GND is generally not practical. In that case, the usual approach is to use ND and GND filters within the same holder system instead of combining a screw-in variable ND with slot-in grads.