So you don't want a discrete (on/off) output, but one that is capable of being set to any chosen value?
Hmmm, let me think. That almost sounds like an analogue output to me. There's nothing on the S7-226 CPU itself to do this, but the expansion module EM232 has two analogue outputs. They'll give you +/- 10V. The only trouble is - there's next to no power from these.
What's your real problem? Why do you need different voltages?
Regards
Ken