Keregioz
Kimon Zeliotis
Again, if we're talking about demos or scratch tracks you're better off sticking with pitch shifted guitar. If you want to use it in an official mix then a VSTi will be a much better choice. Personally I program each note by hand, so no piano knowledge involved. Chances are the playing won't sound as realistic on its own as a real bass (depending on the plugin and how good you're on programming), but the actual sound will be from a real bass so it will sit much better in a mix and give it the low end it needs.