If you have room on the OI it would be eaiser to add a thumbwheel switch there, but here is how you can do what you're asking.
You will form a voltage divider for each output of the rotary switch. The voltage divider is two resistor in series, with one end pulled up to +5V and the other grounded. You take the output from the node between the two resistors.
Code:
^ +5
|
X
X R1
X
|--------------o Vout
X
X R2
X
|
- Ground
=
Vout = 5(R2/(R1 + R2)). So if you wanted it to be 2.5V you would use two resistors of the same value. To be consistent, the total resistance should be at least 100k ohms. You will have to figure out what the standard resistance values are, but heres how you could set up 9 discrete levels:
Code:
R1 R2 Vout
90k 10k 5(10/100) = 0.5V
80k 20k 5(20/100) = 1.0V
70k 30k 5(30/100) = 1.5V
60k 40k 5(40/100) = 2.0V
50k 50k 5(50/100) = 2.5V
40k 60k 5(60/100) = 3.0V
30k 70k 5(70/100) = 3.5V
20k 80k 5(80/100) = 4.0V
10k 90k 5(90/100) = 4.5V
You could also get some small 100k trim pots and tune them for each input. Then you would only need one per value instead of two resistors. They have a small screw that you turn so they shouldn't slip once set. All of this is way more trouble than its worth, try and work the switch into the OI if you can.