Hi Thorn,
The only thing to top of a cartridge with and not lose flavor is
e-liquid. If you are using pre-filled cartridges, then you are correct that you can not match the same flavor. But it begs the question why you would waste money on pre-filled cartridges if you have
e-liquid?
Filling your own blank cartridges saves money and avoids the problem of mixing different flavors when topping off cartridges.
Bottm line is do not buy pre-filled, and load up on
e-liquid with blank cartridges.
Hope that helps
Sun