There are a lot of lignite mines, which have been flooded already. During mining ground water is usually pumped out of the mine, to prevent flooding. So if you stop mining and pumping, the mine would flood anyway. However that would also wash out minerals and cause some serious problems on the "shore" of the new lake. So usually it is better to flood it as fast as possible.
The issue is that these mines are among the largest in Europe. Garzweiler as well as Bełchatów. However for Garzweiler we are talking 1/30 of the water the Rhine has near Colonge. It is bad, but certainly possible.

Sorry, but it is 1/30 of the water which flows past Cologne in the Rhine in a year to fill up the entire Garzweiler mine. So basically diverting the Rhine for a bit more then 12 days. I realize that what I wrote was easy to misunderstand.
But the key here is that the alternative is to keep the pumps running forever. Otherwise you get an acid lake with collapsing walls. Obviously keeping the pumps running also causes massive problems. I am sure they find a solution for Berlin as well.