Adding songs to existing custom playlist

What is the best way to add a new song to an existing user-created playlist? I can’t seem to figure out how to edit an existing playlist on the device (“edit” is grayed out when right clicking on the playlist). 

Put another way, if I edit a playlist in Windows Media Player and then just sync it again will it only sync those edits, or do I need to delete the playlist off the device and sync the updated playlist in its entirety?  Thanks

I don’t use WMP for my playlists, but I would think that you should delete it off the player and then re-sync the newly edited playlist back again.

I think it works just like iTunes. How about using RockBox instead?

@fosterdrew wrote:
I think it works just like iTunes. How about using RockBox instead?

No, it doesn’t. Please heed the advice ini the 1st line of this post:

http://forums.sandisk.com/t5/Sansa-Fuze/Music-files-very-big/m-p/330383/highlight/true#M52044