Shift-M [2+2+2]/4 Enter
The square brackets tell Dorico to use that grouping for beaming and note grouping purposes without reflecting it in the printed time signature.
If it’s just this bar that’s grouped like this, you can input a time signature just for this bar, then another for the next bar, then hide them from the properties panel. Note that these hidden time signatures will break multi bar rests, so if there are other layouts where players are resting, you may want to add these meters independently to this staff. Again, use the popover but do so having invoked the caret on this staff, and confirm the entry with Alt-Enter rather than Enter, which will make the meter independent.