Use custom .M4A as an alert tone in iOS 12
To make a custom ringtone or text-tone:


Open clip/song/sound using iTunes. Use the Song Info options panel to trim the start and stop time if you would like. 
Select the song in iTunes and go to File -&gt; Convert -&gt; Create AAC Version
Right click on the new version and select Show in Finder
Change the extension from .m4a to .m4r
Delete the song from your library. Make sure to not let iTunes move the original version to the trash.
Double click on the .m4r version of the song. It will appear in your ringtone section.
Change your song's start and stop times if you need to reset them to normal.


Since you already have an .m4a, you should be able to make a new copy and change the extension to .m4r and open it in iTunes.

edit It's important to note that to find the Tones section your phone should be connected to iTunes, then go to the overview of your phone, and select the Tones section from the left panel. The Tones section will generally not show up in your normal library.