I've hit that problem too. I just made a 2nd smart group to add the rest.
Thanks, Patrick. I've resorted to that too. Hopefully, HEAT will chime in here with an acknowledgement that this will be addressed.
You are not the first to run into this, but there are several work arounds.
From the sounds of yours, I would use a custom field for Group (or some other name). You can then assign that custom field a value of A for example. Then you find all devices where Group = A. You can import a list of serial numbers to set the group as well.
One of the problems of doing it the way you are is that there is no good means to search efficiently. You have to evaluate many conditions vs a single condition. If you are interested in looking at this, let me know and I can help you.
Thank you, Rob. I'd been so conditioned to use a "Smart Policy" that I'd failed to recognize that a standard policy will actually work better me. As you pointed out, it can be quite difficult to search for a particular device s/n in a long list of conditions. Using a standard policy seems more efficient so far. Thanks for helping me to stop and think it through a bit more.