Would it be possible to make an a building that behaves like a pump for the accumulator wagon? Like a pump, depending of the orientation, it either charges up the accumulator wagon or makes it's charge available to the local electricity network.

Hrmm.. I think that would be possible, and seems like it would be an intuitive way to set up the charge and discharge priority. I'll look into implementing that - in the meantime, you should be able to mostly get the behavior you're looking for with the custom circuit signals - at stations where you want them to charge connect a combinator sending the special "Charge accumulator wagon" signal to cause them to prefer to leech power, and at stations where you want them to push their charge into the network use the Discharge signal.

Now that I think about it, it wouldn't even have to be dependent on the orientation. It might as well just be implemented as two separate buildings, a charger and a discharger. The discharger might as well connect to power poles, then.

Agreed. There's a feature coming soon in 0.17 that'll make the building simpler to implement, so once that's out I'll see what I can come up with!