Some manufacturers say you can't, but from a technical / electrical perspective there is absolutely no reason for it.
The reason it is sometimes frowned upon is that having extra outlets could mean that too much is plugged in and the UPS is overloaded, just like if you use a power board on a normal power outlet and overload it.
So Yes, you can plug a power board in, but be mindful not to overload the UPS. Also it pays to make sure that the UPS power board is out of the way so no-one can plug a vacuum or heater into it.
