The good news is that Checkmk makes augmenting an existing check pretty easy. The monitoring home has a local folder and you can pretty much mirror the whole setup (well, just the pieces you need) there. You can also “add” instead of just copying overriding. Just make sure you give new names to everything in your modified plugin structure.
See also:
Once you have “your modification” or “new plugin” checkmk has a plugin creator that can take your modifications and bundle it into a .mkp for redistribution (this is optional of course, but useful).
Alternatively, though a bit heavier, you could code up a simple check, but would run locally (or somewhere) and do an snmpget on your specific value and format, etc.