Hi, and welcome to ECF and welcome to the world of vaping! When the vapor starts dropping off a little, you should top off your cartos. If you wait until they taste burnt, it will be too late because you'll never be able to get the burnt taste out. Yes you can add FSUSA liquid to the prefilled cartos, but it will definitely change the flavor because the two liquids (the pre-filled stuff and the new stuff) will definitely blend. I'd suggest that you use a "similar" juice. For instance, if the carto is pre-filled with a tobacco flavor, you can add another tobacco flavor, or maybe a coffee flavor. If the pre-filled carto is filled with a fruity flavor, you could add a different fruit flavor. Otherwise, you would need to clean them out first. In my opinion, cleaning cartomizers is a pain! I don't mind blending a couple of flavors, so I just add a similar juice on top. Others may have different suggestions.
I think you'll be able to "feel" when the cartos are getting low. You can pop the top off and look at the filler. If it's totally dry, I'd just add several drops (12-15) until it looks moist again. Good luck.