There is not an easy solution to this. The problem is that skillsys takes over growths and leveling up. It’s significant work to inject your own routines into an arbitrary hack. It’s easier to just take over things (which will probably break skills like blossom and passive growth boosting items, but I imagine if you’re using negative growths, then breaking those things isn’t really a concern). But more importantly, a separate magic stat does not exist in vanilla, so you need the address of the character and class magic tables, at the very least.
For my installers for the self randomized hacks, I first insert a blank .event file so febuilder will generate a .sym file for the required addresses to define.
#define MagCharTable 0x8B2C124
#define MagClassTable 0x8B2C324
#define Skill_Getter 0x8B4ED44
https://github.com/Veslyquix/SRR_FEGBA/blob/main/HackInstallers/FourKings.event
With a similar event file made, the next step is to edit/write some C code that checks mag growth and levels down stats.
CheckBattleUnitLevelUp:
https://github.com/Veslyquix/SRR_FEGBA/blob/36be74dcd15c9a2eebc0bc2d567d46462eb08a90/C_code.c#L11689
Mag getter related:
https://github.com/Veslyquix/SRR_FEGBA/blob/36be74dcd15c9a2eebc0bc2d567d46462eb08a90/C_code.c#L8748-8943
My code is a relevant example, as it includes configs like 0% and 100% growths, but it does not include negative levels.
To inject code, you could follow this:
A wizard could do all this, but even then, it’d take a lot of work.