I was looking for the solution to this and came across this thread. One thing led to another and I figured out a simple solution.

Go to the Final Cut Pro app, right click and select “Show Package Contents”.

The navigate to Contents–> Plugins–>MediaProviders–>MotionEffect.fxp–>Contents–>Resources–>Templates–>Effects.

There I made a new folder called ” Favorites” [notice the space before the F] and copied several effects into it.

I then relaunched FCP and there is it, a Favorites Effects folder. There is probably a bit more we can do using this method, this was just my first try.
Hope that helps.