From what I can tell, the SI-240D expects an analog input signal. The closest that you can get from the PMDX-122 is a PWM signal from Mach3. If the SI-240D has any kind of integrating capacitor circuit inside, that could work. But I don't see any mention of it accepting a PWM input.
The 0-5V output from the PMDX-122 is a digital output, i.e. either close to 0V or close to 5V. Nothing in between.
If you had PMDX-126, you could add a PMDX-107 to drive the servo controller signals. With a PMDX-122, I am not sure you can get there. Well maybe so - look at the PMDX-407. There appears to be a PMDX-407-S version that claims to work with the PMDX-122. If that is available, you shouldn't need the SI-240D as the 407 should connect directly to the 0-10V analog input on the servo driver.