Unfortunately it is not possible to increase the magnification to more than 10x with the official firmware of the Canon 6D. The only possible are actually 1x, 5x and 10x.

I'm not even sure if it is possible to create a firmware capable of magnifying it even further. Looking at the resources at Magic Lantern here and here (an unofficial firmware for the Canon DSLR:s) it seems like the DIGIC has calls for the different levels of magnifications. If these are implemented deep in hardware it may very well be impossible to create new levels of magnification performance wise even if a custom firmware is used. I have however not exerted myself to read a lot about these functions in the DIGIC processor and there may be a way around this.