Shimano 7 speed top bar thumbshifters had an 8th position - and could be used with Shimano 8 speed drivetrains. I do not know about SHimano 6 speed DT shifters...

It is quite likely that shifters do not have a 'hard stop' (physically stopping the shifter from going past the last click), so if the bike is in the 6th cog, simply pulling the shifter past the click will move the derailleur into the 7th cog. In this case, the end-point adjuster screws on the derailluer will serve as a hard stop and a final index position.

The real problem, alluded to above, is that the spacing between the gears is different. If the spacing on the 6 speed is 0.3 mm larger than the spacing on the 7 speed, the derailleur will be 1.8mm off after the sixth shift.... this may or may not be enough to make it index badly.

Another option is to try to change the cable actuation ratio by moving the anchor point on the derailleur. If the cable is clamped between two washers a few mm away from the normal pinch bolt, it could make the derailleur move a little less for a given amount of cable pull.