By PEX you mean Processor Expert? Actually - SafeRTOS isn't supported by S32DS and processor expert. As far as I know, MPC577E itself is not supported by SDK and ProcessoExpert.
Actually FreeRTOS component is available in component library in S32 IDE, which can be added to the project with target processor MPC5775E.
Also some more components like I2C, SPI.. can be added to the project and code can be generated using processor expert.
My query is whether I can able to replace FreeRTOS with SAFERTOS or not?
my bad, I figured out that MPC5775E is supported, but there are no examples for this device yet.
Technically - you can use SAFERTOS, but not as a part of Processor Expert. Just remove FreeRTOS component and add your SafeRTOS code into project.