Please find the answers to your questions below:
NetX can be added to the firmware as long as you purchase the license. All of the ThreadX OS services are part of the library that we provide, and nothing has been filtered out. So, there should be no problem with getting NetX to work.
We can only provide ThreadX in binary form, and are not authorized to make a source release. We can give you the version information and build instructions, so that you can purchase your own license and compile the OS in your environment.
If you are using the OS in the library form then you do not have to pay any licensing fees or royalties to Express Logic.
Could you please let me know what version of ThreadX the FX3/FX3S uses? Also, if you can provide that build instructions too that would be great.
The threadX version we use is 5.1 (dated December 2007). I will send you some info on the build by Monday.