It’s possible in FE8 due to the ExModularSave patch. If you want class extension in FE6/7 you could use ExModularSave as reference to write a version for FE8, and then potentially have to change a couple of functions where it reads a half-byte to read a full byte instead.
But no, it’s not the sort of thing you can easily do with just pressing a button. If you’re not willing to learn more advanced topics such as “hex”, ASM and EA, you should make do with existing class slots. If you are, though, I suggest starting by reading GBAFE Assembly for Dummies, by Dummies, experimenting with FEBuilder’s Disassembler and looking into ExModularSave’s implementation with Event Assembler.
EDIT: And I guess it’s just proper to recommend hacking FE8 instead, if what you’re making is a custom hack rather than a reskin of FE6/7, due to already having the possibility you want, more functionality in vanilla and much more support with patches and documentation.