You don't mention which version of Sonar you are using.
However, look at the set of icons directly above the tracks in Track View. I believe in S3 and earlier there is an icon labeled "offset mode." When you click it, it will toggle into offset and all your pan and volume settings will read zero. When you change a volume (or pan) setting in Offset Mode it will read +1, +2, or -1, -2, etc. Basically what this means is that you are adding (or substracting) from the original setting.
So, for example, if you have a track with a volume setting of -3 db, and you go into offset mode and set a volume of +2 db, your volume setting for that track will now be offset by +2 db. When you toggle back to the normal mode, the volume will still READ -3db, but since it is offset by 2 db, the actual volume will be -1 db.
It should work fine for what you want to accomplish. The only drawback is you will need to remember that it has been offset - since unless you toggle into offset mode, the offset is invisible.
(BTW, in S4 Offset Mode is found on the dropdown list off the Envelope Tool icon.)