Skeetendo

Yes. It is possible, with this tool I made called The Soul Sucker 9001. What you do is you open a new file in a hex editor, enter certain spiritual characteristics (like a happy spirit, a vengeful spirit, the kind of spirit that will steal your face if you show any emotion whatsoever, etc.), and save. Then you use my tool to convert the spirit into a Pokemon compatible spirit file, and enter it at the right hex offset. I think there's a list of hex offsets somewhere around here...