Yes, you can use a solid ND to darken part of the frame, but it won’t truly behave like a graduated ND. Because the filter is very close to the lens, its edge will usually be out of focus, so the transition may blur somewhat rather than appearing razor sharp. However, it will still generally be harsher than a real hard-edge grad, which has an actual gradient.

The bigger concern is image quality differences between the filtered and unfiltered areas. A thicker plastic or glass filter can introduce focus shift or slight magnification differences across the boundary. Thin gel-style filters are less likely to cause that.

This setup is better thought of as a partial-frame ND effect, not a true graduated filter. It also won’t work well with round screw-in filters; it’s more practical with square or sheet filters that you can position precisely.

So: yes, it can work in some cases, but expect a less natural transition and possible optical issues compared with a proper graduated ND filter.