That was done with other game hacks (gecko os), not ACToolkit. ACToolkit is for modifying saves, not active data. There are codes for Wild World that force slots in your inventory to ALWAYS be a certain item, and those have been translated over.
As for "rare star grass", it doesn't exist in normal game data. I heard there were a few odd grasses in Wild World (in fact, there are AR codes on this website or on the net to access them). Dunno if the same odd grass and snow patterns and colors are here, but... the folks here usually don't skip useable features.