No adding true walkman to a k810 / k800 / k790 is at the moment impossible.

The only way to actually do this would be by crossflashing but then we get all kind of problems. If we were to crossflash a k810 to i.e w880 atleast i would not consider the phone a k810 anymore. So adding walkman to k810 no can do ;)

What is possible though is using KDplayer or gfx patch the original music player.