Mewtwo Strikes Back
4. Go to the sprite you want to edit in Spread, and pixel by pixel change it to the new one.
This may take some time, and I'm sure there is probably a program to insert new sprites, but this is how I do it.
Importing the sprite would be a lot easier. BTW, The bitmap import/export function was implemented (and improved from the original one) since the first release of Overworld Editor RE.