First, you may not need to do that since the precision of intermediate files and calculations means you can raise the volume at the final bus and get numerical results that are clean.
But to do what you want, you can group all the audio tracks as you did, switch into "offset mode", raise one of the faders, then switch back out of offset mode (back into what is called "envelope mode"). Offset mode operates the track sliders as offsets added to whatever is happening in envelope mode. So it can work like changing the automation envelopes by a constant. Offset mode is accessed by the offset mode button in the Control Bar’s Mix module.
Check out the manual. Here are some excerpts:
There are two modes which control how your volume faders, pan faders, bus send faders, and bus send pan faders behave during playback. The two modes are Envelope mode and Offset mode....
In Offset mode, all controls that can be offset appear with a plus sign. For example Vol+.