Okay this comes form the official service book for JU88A4:
Every delivery package of a JU88 delivered to Finland consisted of a single 20mm MG FF. It was not a field modification but a part of regular armament options.
You either had the MG FF OR the level bombing sight (Lofte, BZA) in the plane.
The cannon had either a 60 round or a 90 round ammo drum.
If the back of the cannon was locked into place the firing mechanism was connected to pilots controller so he could fire it using a reflector sight, or if the observer unlocked the gun he could shoot it within a rather limited arch.
The cannon armed JU88s were used in Finland to attack troop transportations and shipping.
So, it was a standard armament option for A4.
Needless to say but I'd like to have it.
-C+