Hello Ivaylo,
You could load a host controlled device on the KW41Z, connect the boards through a serial interface (UART, I2C, SPI) and communicate the devices using THCI commands over FSCI. See Chapter 3. Host Software Overview in Kinetis FSCI Host Application Programming Interface.pdf.
As you already mentioned, you could use the K64F + MCR20A as a standalone border router, there is already demo project for that in the Connectivity Software package. In this approach, the Connectivity Framework and Stack is handled by the K64.
-JC