You can combine multiple frames to mimic some long-exposure effects, but it is not the same as a true long-exposure photo.

The main limitation is resolution: video frames are much lower resolution than still photos, so the final image will usually have less detail. Video also uses whatever shutter speed and processing the camera applies during recording, so motion blur and image quality may differ from a real long exposure.

A better approach, if your camera supports it, is photo stacking: take a series of still images and combine them later. That keeps the higher resolution of photo mode while letting you build up the effect over time.

If your camera has a remote port, an intervalometer may help by automatically triggering repeated still shots for stacking.