So, this "original" PWM comes from somewhere else, and that's all you have? You can't get to the raw data creating the PWM? That signal is PWM and not a servo control pulse, right?
You can filter PWM to varying-DC with a low-pass filter. For servo speeds, a simple RC Filter
should do the job.
Run that DC into an analog input (I'm assuming 5V PWM?) and convert the A/D reading to an angle for the servo library. The map() function is one way to do that.
I'd probably start with porgramming & testingt the servos under software control, then work on converting the PWM to an angle, then combine everything.