Others looked great. Most came with wonderful documentation. Like many such products RTXC comes with complete C source. Though an RTOS is conceptually pretty simple, real products offer so much functionality that their code is substantial and non-trivial. I quickly decided I had no interest in learning how it worked, so decided to just include the code with my application and not make changes to it.
Many users expect the source for maintenance purposes.
This makes sense to me as insurance against the vendor's demise, but I'd be hesitant to make changes unless there was an overwhelming business reason to do so. Any change will make your version incompatible with future upgrades, and will probably invalidate your warranty. Make changes in your application to deal with the peculiarities, if any, of all purchased software modules. RTOSes are little more than a collection of services that you invoke in some manner. Some use software interrupts to isolate the OS from application code - you issue the interrupt, passing parameters.