Yeah it should be possible with some header/extheader edits and rebuilding the cia.
Like KleinesSinchen says, space is limited. Iirc it's about 600MB for old3ds and about 1000MB for new3ds. I think there was an app that allowed resizing ctr-nand to fit the later 2GB nands, but I forget the name. (could be gm9).
Experimental nand mods like this are not recommended at all though.