Could a road bike be used for cyclocross?
Depends on your road bike, but for the most part no.

Reasons against

Many road bikes are limited to tire width of 25 or 28mm.  Because cycle cross is off-road you need to find tires with knobbies. This really isn't possible for such small tire sizes as most knobby tires available start at around 32mm or large. Even if you could find knobby tires this small, The tight tolerance between the tire and the brake caliper will mean any mud would clog and slow the wheel from moving.  Finally, most rode bikes have a much lower bottom bracket and twitcher handling. The lower bottom bracket will mean a lot a pedal strikes especially on off camber sections and the faster steering will make this type of road bikes harder to hold a line in rough sections. Cyclocross bikes also tend to have lower gearing, a much slacker front geometry and longer wheel base so that the bike hold the line better. The higher BB not only reduces pedal strikes but also makes it easier to initiate change in directions by shifting weight, despite the slower steering.

Reasons for

Some road bikes slacker geometries and more tire clearance (e.g.,older, endurance and adventure road). I had one steel bike that could clear 35 mm tires, this one could have been used on dry courses. The bottom bracket will still be too low and the handling likely a little too twitchy, but these would be much more suitable than what most manufacturers  currently sell as race road bikes at the moment.

Note that many cyclocross series do not follow UCI regulations and are happy to let you race whatever bike you bring.