You can add multiple song and artist seeds from the station list on your device.
 
First, tap the station you would like to edit.
 
Then tap on ‘Add Variety’ underneath the station name*. If the station automatically starts playing, tap the back arrow in the upper left hand corner to return to the Station list.
 
A window will then open where you can add more song or artist seeds.
 
While typing in your selection, the search should auto-complete; however, you can also press the blue "Search" button once you're done typing to bring up a list of relevant matches.
 
After the list of stations pops up, select the name of the music or genre station that you want, and you're good to go!
 
*Note: ‘Add Variety’ is not a feature of Genre or Shared stations