You can only add grades that have a sales history on the marketplaces we monitor. There are a number of issues that arise if we allow our users to add new grades that we do not currently support. If you cannot find a specific grade, you can always save the card as “Other” and add a note.
UPDATE: We are working on allowing you to save any grade from the major grading companies (PSA, BGS, SGC, CGC). Coming soon.