MK61 belongs to K70 series and NXP does not support it in SDK. Currently no plan to support it in SDK.
One solution is to use MQX 4.2. which has MFS driver that includes support to Nand Flash and DDR memories.
So we can create a new BSP for MK61 based on TWR-K70F120M, which has a MCU quite similar to K61,
you also can use bare metal project.
I'm also having the same issue. I was searching SDK for the TWR-K70F120M almost everywhere. Now when I read you are not planning to support K70 SDK I'm really disappointed. I don't know how I can work now!
For bare metal code, please download it from below link
For MQX, you can download it from below link. K70 is very similar to K61.