Hi Calvin:
LWIP can be as a standalone stack, you can download the SDK for imxRT from below link,
Welcome | MCUXpresso SDK Builder
You will see some lwip demos in this package, some are based on freertos, some not. Let's take rt1051 for example, the lwip folder is:
SDK_2.5.0_MIMXRT1051xxxxB\boards\evkbimxrt1050\lwip_examples
Regards
Daniel