Welcome to the community Zane 
As far as I know this feature was first requested in the below topic & here’s Hugo’s explanation of why this hasn’t been built -
https://community.monzo.com/t/custom-categories-1/649/14
I’d like to be able to do this too, if OCR of receipts was possible or the itemised breakdown of receipts could be fed through to Monzo from the merchant, that would make the functionality much more user friendly…Yoyo are working on a system which provides the latter.