Using the right-click "Merge Selection..." context menu in a Track Pane, or the "Edit/Merge Selection..." menu or toolbar icon will merge two or more selected tracks into a single track. The segments from all selected tracks will be preserved as segments in the combined track, and sorted by time.
When merging tracks, you will be prompted for the name of the new track.
If the selected tracks have overlapping times, the combined track may be ill formed.