No problem. Just pour the chlorine (regular bleach) slowly near the return jets. Check your FC again a little later to ensure you are holding in your target zone. FC/CYA Chart If the FC is dropping too fast each day, more than 4 ppm in 24 hrs, consider doing an Overnight Chlorine Loss Test.
Shouldn't matter if the SWCG is on. Just like adding any chemical, you'll want to do so with the pump running and in front of a return to disburse into the water and follow up with a quick brush around.