Usually you get that "select -> paste" functionality by selecting then clicking the middle
mouse button (that is, mouse button 2). If you turn on "Emulate3Buttons" (I believe that's right, if not then it's called something very similar) in your X server configuration then you should be able to do it by clicking both the touchpad buttons simultaneously.
Note also that the above will apply to any X environment, not just KDE; so maybe it's not what you're looking for after all. I only use FVWM2 and hence don't know much about big fancy desktop environments.