Look up the “Fit to Fill” function in the manual. I’m not sure this is exactly what you are looking for, but it’s a valuable function to know nevertheless.
Basically set an In and Out point for your clip in the Viewer, and set an In and Out point in your timeline where you want the clip to go. Drag the clip from your Viewer window to your Canvas window and drop it into the Fit to Fill box
Final Cut will automatically adjust the speed of the clip to fit between the In and Out points you selected in the timeline.
For example if you had a 5 sec clip and an 8 sec gap in your timeline to fill, Final Cut adjusts the speed of your source clip so that it fits perfectly in your timeline. It also doesn’t move anything else in your timeline. Make sure your audio tabs (A1, A2, etc) aren’t selected.