Yes, what county? Simplex fireground frequencies are usually licensed to specific departments or counties.
MARCS does have three 800 MHz simplex frequencies (MARCS TA-1, TA-2 and TA-3) used in P25 mode which are listed in the "MARCS Conventional Frequencies" section of the database, but NFPA recommendations call for critical simplex fireground use to be analog.
If they're not listed in the database for your county, and you're close to a working incident, I'd suggest searching between 851 and 861 MHz, or using your signal stalker/close call feature (if you're really close).
And then submit them to the database.....