You should look into Process Expert. It allows you to configure the TARGET with a drop-down-based GUI rather than hand-coding the bits.
To create the TARGET app:
- Create a new KL25Z PEx project
- Add USB component
- Add serial component
- Swap the data however you'd like via user code
- There are specific clock settings required for USB to work; a Google of "KL25Z USB Clock PEx" or similar should return a relevant tutorial
I'm willing to bet that PEx comes with a ton of example code, as well.