XPS 8940 and 8950, add pwm fans, control the speed with a hub/program

One "gotcha".  Dell uses proprietary fans that interface with the BIOS.  Non-Dell fans can be used but because they are not sensed at POST (Power On Self Test) you will always get a fan failure notice and have to press an F Key to boot. 

I've seen references to Dell fan wiring and I remember something about non-ATX wiring so something else to look into. If you use a separate fan power distribution unit then this is not a problem.

There are other posts (threads) about fans in these models.  Do a search for them and see what was done or couldn't be done.

Surely you can, but there are prerequisites,
with SW control approach your application should be capable of:
1) reading temperature signal from CPU/VR + PCH and Ambient (can be done via relevant MB systems)
2) reading tachometer signal from CPU/Sys (et.c) fans (can be done via relevant MB systems)
3) sending control signal to right PWM systems (CPU/SYS/Chipset)
It's normally implemented via SuperIO controller (SMBus/I2C)
If your program can interact correctly with chip (read temps and tacho signals, write/set PWM override values) - then job is practically done, just install right fans/connectors/splitter and configure application.

Simple hardware approach might be much easier to implement without trying to programmatically find right port/address/offset/bits to read/write (or compatible application what could interact with your specific SuperIO controller). There are various 4-Pin PWM signal splitters available (tacho/sense read from single fan but PWM signal sent to multiple fans, aka power-share) for cheap (simple cable, with multiple outputs, or more complex circuit boards with additional power amplifiers/stabilizers, indicator panels, manual control resistors, separate thermal sensors and individual PWM chip, etc.).

As @fireberd has mentioned - Dell is very sensitive with installed devices compatibility-wise, once I've installed additional system fan and it has messed up whole balanced automatic regulation by internal logic, CPU temps started reaching 90 degrees without fan kicking-in to higher RPM (leading to thermal shutdowns), or CPU going into Turbo-Boost mode for prolonged periods of time without any need (e.g. without any system load), and whole range of other anomalies and instabilities, in the end I had to disconnect one of 4-pin connectors (Sense/Tacho pin) to get it back to normal operation without confusing internal PWM/thermal control logic coded by Dell.