According to the Wikipedia page, Dragon 2 has a (pressurized) return payload capacity of 3,000 kg. This is definitely enough to bring back a few extra adult humans (though you may have to remove other important payload that was initially meant to be returned to earth).
Your question however asks whether the extra astronauts would be able to hold on for dear life and still survive reentry
As @MikeH mentioned, the extra astronauts would not have custom SpaceX suits, made specifically for the seats and pressurized to ensure astronauts don't die during emergencies.
Having a suit that integrates with the seat is of course a good thing to have, and provides confort and safety, but if an emergency happens, you could be strapped tight to the seats, perhaps wearing your Sokol suit (that you would have used in the Soyuz) for extra protection, and that would suffice for reentry.
Of course this could cause problems if the capsule depressurizes during reentry as the Sokol suits are probably not compatible with the Crew Dragon, but while it takes about 35 to 40 minutes for Dragon to re-enter the Earth's atmosphere and splash down in the Atlantic Ocean, entry into the atmosphere itself where danger is the most likely only lasts a few minutes, and the atmospheric pressure after that is survivable.
Now what of the astronauts left without an actual seat?
Well, Space Shuttle contingency mission STS-400 (contingency for STS-125) would have launched with 4 astronauts and returned with the 7 from STS-125, for a total of 11 astronauts in a single orbiter. All 7 STS-125 extra passengers would have strapped in on Endeavour's lower deck, through 'lay on the floor' kind of seating.
I believe something similar would be possible to secure other astronauts in place in Dragon's payload compartment.