Hi Douglas,
You can navigate to Projects -> C/C++ Build -> Settings -> MCU Linker -> Managed Linker Script and check the "Link application to RAM" checkbox, make sure that the External DDR RAM is the first RAM section defined in the MCU memory configuration. Please notice that you first need to ensure the external memory controller is setup so the debugger can access the SDRAM. MCUXpresso doesn't know how to do this for you. One approach would be to write a MCUXpresso connect script for this purpose. Connect scripts are configured in the project launch configuration.
Hope it helps!
Best Regards,
Carlos Mendoza
Technical Support Engineer