Should be possible to add a pickup (route a hole, drill through to the control cavity to run the wire) and replace the 3 way switch with a 5 way that fits into the same opening. You could keep the same switch and add an on-off switch for the middle pickup

EDIT: Doge is right, of course- you'll need to either look at an active pickup or replace the current ones if you want them to match in any usable way.

Since the EMG 81s are active, you are going to struggle with space and possibly the drain on the batteries. You can't really match a passive pickup wth actives as they require completely different tone and volume impedances. Your best bet would be to change all the pickups to passive or you can try put in a third EMG 81 or something similar hoping that there's enough space.

Oh I didn't even think of that. Damn... Oh well.

Thanks guys.

No point in adding more 81s, rather go bridge 81 mid SA neck 89. Don't worry about the battery, not a problemo.