If you do not have enough free space in the ROM you want to hack, that will be impossible.
Free Space Finder
Type: Miscellaneous
Compatible with: Pokemon Ruby, Sapphire, Emerald, FireRed, LeafGreen
Supported OS: Windows
Download Pokemon GB GBC GBA NDS Hack Tools
With a simple-designed interface for even a beginner can use, this tool can change the number – the name – the description – the special values of that item.
Have so many different Pokemon is good, but you have to notice their icons. Change them with IconEditor, a practical tool for editing Pokemon icons.
After finish editing the Pokemon icons, let’s come to the Item icons. Change them with the Item Image Editor, feel free to do so.
Not only Pokemon FireRed can be hacked, adjusted with a tool but also Ruby – Sapphire – Emerald – LeafGreen… all the games of Gen III.