I was wondering whether it would be possible to turn a synth/electronica track's keyboard line into a bassline. I'm thinking of turning a load of Depeche Mode tracks into basslines, and there's a guitar part but no bassline other than the keyboardist's left hand part.

How do i do this?

find out the notes and transpose it for bass, or listen and try and play it by ear.

a nice synth line on bass is the intro line to 'don't you want me baby' by human league...i think that was keyboard rather than a synth bass anyway.

Bass is bass no matter what instrument is playing it.

That is what caused the resurrection of the 5-string bass. Bass players needed a away to play the low parts that synth bass had been doing. Just transcribe the synth bass like you would any other bass line. If you don't have a 5-string bass and the parts goes below open E-string then you will have to move them up an octave.

Many of the studio bass players these days double on synth bass, work is work.