@Dark it seems to me that it was regulated by a byte, in the RAM where the unit is saved, however I am pretty sure that it can only be used once because of this, unfortunately I am not an expert, and I could not tell you anything else
however there are several ways to modify the growth rates of a unit, you could also create a scroll as in FE5, or through a skill, I don’t think it was difficult, quite the contrary, the only thing, it seems to me that maybe FEbuilder doesn’t allow to do these things directly, and I think you should go through a buildfile
EDIT:
since I was trying to practice using (and understanding) the debugger I started to see just a little bit for: then usually the location of Eirika’s memory is in this position 202BE4C, looking at Tequila’s notes a bit you will find that the byte that regulates what you are looking for would be 0x0D, therefore: 202BE4C+0D (the value 0x20 indicates the use of the metis tome)
yes apparently I was right (wow I’m surprised at myself)
I suggest you go and see Tequila’s notes about the assembly are very useful