Just to add to Johns reply, which is right on, it might be better if you told us what you want to accomplish, then maybe we would have a better understanding of the challenge you are facing. In the old days of analog or linear editing, you could actually cut and splice partial frames, or use the editing hardware to insert a segment part whay through the frame, but the software products that are based on non-linear editing require edits to be made at the frame level.
There are a few ways to get around this, but it would make it much easier if we knew exactly what you are trying to do.