I think you could increase in a couple of ways. You could continue the raglan increase rows as you work from the top down for one more row. That'll increase the sleeves (and also the back and fronts). Then you could increase 2sts on each side in row one. This may be a good way to go since your gauge is so much smaller (more sts per inch).
Or you could increase 2sts on each side in row one of the sleeves and then increase one st on each side in row 2. Either will give you 4 extra sts and extra fullness in the sleeve at the upper arm.