Is there a motorcycle parking in Belgium?

As for as I know, there isn't any motorbike parking at Charleroi. Personally, I'd never leave a bike there anyway as the probability that it'd be there when I got back is relatively low.

The FAQ for the airport does say that bikes are not allowed in the car parks (see below) but that there is free covered parking. As a regular user of the airport, I have to be honest and say I don't recall ever seeing it.

Y a-t-il des places pour les motos? Dans les parkings, il n'y pas d'emplacement prévu pour les motos. Les motos ne sont pas autorisées. Un parking moto gratuit a été prévu à l'extérieur, à côté des bureaux Rent a Car. Ces emplacements sont couverts. Suivre les indications Rental. Il n'y a pas de réservation possible.

It’s mega expensive to park theirs even if you book ahead could you get the airport bus or a lift?

It is absolutely possible, no issues at all. The bikes go to P1 which is the closest to the terminals and is dry as well. Bikers also don't pay parking fee . There is no specifically arranged spot for the bikes though. So you have to figure out for yourself where to leave the bike. I left mine in a blind corner where no car can fit.