Hi Abhi,
Sorry for the late response!! I had some issue with my IAR license until I re-installed it. I have implemented the CDC demo running in DDR based on TWR-K70, but It is not only the link file, but also the PLL init function called in the usb stack should be modified to make the usb stack run in DDR.
First , I recommend you start with a SRAM target configuration,

second, just as mentioned before, use mac file to initialize the PLL0 and PLL1 as well as the DDR controller before the IDE load the image.

and choose verify download if you like

then, you may change the sram link file as the following:

you may find the RAM section are in DDR now, but please note don't set 0x08000000 to 0x10000000 as the range, this configuration are used by KINETIS120MHZ_SC, but as the USB stack uses DMA to transmit or receive data, you have to put the section in a location that allow access from both core and eDMA.
and set the vector table in SRAM.

As PLL0 and PLL1 have been initialized in the mac file, there is no need to do it in SYS_Init(), add some code from line 884 in main_kinetis,c
#ifdef SDRAM
/* Initialize clocks to match values from init script */
mcg_clk_hz = 120000000;
pll_1_clk_khz = 150000;
pll_0_clk_khz = mcg_clk_hz / 1000;
#else
#define ASYNCH_MODE /* PLL1 is source for MCGCLKOUT and DDR controller */
#define SDRAM
and #endif at line 924.
Define SDRAM in "derivative,h "
#define SDRAM
After that, run the application , you will see the following view:

and found the cdc device in the device manager:

Hope that helps,
Have a nice weekend!!
B.R
Kan