Side Chapters on a FE8 rom

As someone with not that much knowledge of using FEbuilderGBA but slowly trying to figure it out at my leisure, I’d like to know if it was possible to make side chapters in a romhack made using FE8, much like the ones in FE7? (ie. Protecting a bunch of green units until the end of the chapter will have you play a secret chapter before the actual next chapter)

Sure; just have a check in your ending events to see whether the condition(s) for the side chapter have been fulfilled, and have a different MNCH (move to next chapter).