With this tool, you can even add 4 different maps and make a big Pokemon world of your own.
Positron World Master
Type: Map Editing
Compatible with: Pokemon Ruby, Sapphire, Emerald, FireRed, LeafGreen
Supported OS: Windows
Download Pokemon GB GBC GBA NDS Hack Tools
Not only Pokemon FireRed can be hacked, adjusted with a tool but also Ruby – Sapphire – Emerald – LeafGreen… all the games of Gen III.
The tool Nameless Tile Map Editor (NTME for short) is created for editing – adding the tilemap for Pokemon Gen III games.
If you think your overworld is too small or too big and want to resize it to your conditions, use Overworld Changer.