Sure, create a smart playlist in iTunes, then sync it to your iPhone, then in Watch settings select this playlist as the source of its songs. iTunes can create a random playlist by you creating a smart playlist:
- Define the criteria that will generally select music you like (or avoid what you don't). This can be song rating, album rating, genre, last played date, etc. It is up to you to shape the population of what you want. Even if you don't have anything to narrow down the selection, just select all music with something like "Time - Greater Than -- 0:01".
- IMPORTANT: In the smart playlist, select "Limit to" to either a number of songs or max GB, then "selected by random". You need to do this to keep the size of the playlist small enough to fit on your Watch.
- Optionally, include criteria like "Last Played is not in the last 7 days" (or whatever amount of time). That will cause the playlist to update to remove songs you played and replace them with others (but only if you re-sync your iPhone to iTunes).
Sync this playlist to your iPhone -- the Watch can't sync directly from iTunes, then set your Watch's Music setting to use this playlist as its source of music.
