Yes, you're right - it's not a new word at all. I'm not even sure it's a misspelling either - it's just a phonetic spelling of a mis-pronunciation of a word, in this 'something' (or 'summat' in some circles).Agreed.
Sumfin is literally just a word being misspelled. It doesn't mean anything different, does it?
If not, we then need to add all misspellings ever. Daft.