I don't think a country's Olympic committee can just show up in Beijing with more athletes than expected. The extras would not be accredited for the Olympics or accommodated in the Athlete's Village. They could, of course, try to make private arrangements for the extra athletes, but that could be very difficult and expensive.
Some countries in the past have made arrangements for the Olympic athletes to train before or between events in a location near the games, but that could be extremely problematical these days. Imagine trying to get training facilities/accomodation/entry visas for North American athletes to train in, say, Japan. It might happen in an "ordinary" Olympic year, but is extremely unlikely under COVID.