I’m a new user and am wondering if a file can be added to the end of a file that is already loaded.
You can “import” as many audio files as you like, and they will each be imported into new tracks, one above the other (and will all play at the same time 
)
To make them play one after another, you need to slide them horizontally so that they don’t overlap. The quickest way to do that is:
- Ctrl + A (select All)
- Tracks menu > Align Tracks > Align End to End
You can also slide track using the Time Shift tool.
It is not normally necessary to merge the tracks into one track (that happens automatically when you “export” the finished project), but if for some reason you do need to, then:
- Ctrl + A
- Tracks > Mix > Mix and Render