you have to clear this question with Belgium because they made the entrance ban to EU and only they have right to cancel it or reduce the period of time. Germany can't cancel the ban and won't give you a visa for family reunion or anything else before it is eliminated.
That means that you should apply in Belgium, probably in the last responsible migration service (like Ausländerbehörde in Germany) for cancelling the ban or at least reducing the time. At the same time you must clear the question of costs of deportation and pay them. Normally, it is the condition on which Belgium would cancel the ban. You should probably make an application for setting the amount of these costs and, if these are to high to pay at once, ask for an agreement to pay in installments. As far as I know, German authorities agree sometimes that people pay a half, for example, and the other half in installments, when they are already here. But some people must pay everything. How it is in Belgium, no idea.
You should also check, if you really fulfil all requirements for a German visa for family reunion (after cancelling the ban). In most cases you must have at least A1 certificate. You must have enough money as a couple not to ask for any help from the state and enough living space for 2. Perhaps there is something else, it can depend on the kind of your wife's residence permit.