There WAS a 20 or 25 round NON-detachable box mag (actually a magazine extender) issued in WWI for Mausers. The 'Air Service Rifles' that the US made during WWI by modifying 1903 Rifles also had 25 round non-detachable magazine extenders. Original ones of these are both pretty hard to find anymore. Copies have been made of both extenders, but you don't stumble across those every day, either.
The way the floorplates latch into the trigger guards and the fact that the magazine feed lips are milled into the receiver is going to make the adaptation of 'standard' Mausers, M1903's and M1917's to use detachable magazines without EXTENSIVE modifications a pretty difficult proposition, at best.
On the other hand, someone who is reasonably handy with metalworking would be able to cook up a working, non-detachable, magazine extension that attaches like a regular floorplate without a whole lot of trouble.