If I was gonna add nic to already flavored
juice, I'd wanna add high strength nic base. Your best bet would be 100mg which wouldn't dilute the flavor too much at all I don't think. Adding 1mL to 9mL flavored
juice would give 10mg strength, or for higher nic adding 2mL to 8mL would give you 20mg strength. You could get a small bottle of nic base from 1 of the sites like wizard labs or vapor shots for less than $10, but you're definitely gonna need to get a syringe too for measuring.