You can pop off the cap at the mouth end (if it's a soft one - the blanks and some of the premos and all of the reg cartos at V4L are), tilt it like a beer glass, drip down the side as you turn it, and get *lotta*
juice in.
You can also use the "condom method"... just drip into the condom, and slowly sink carto mouth end in there until a drop appears at batt end. Carto sucks it up. Wipe mouthpiece off, ready to go.
AND, if you spin it (search Taryn spin in the forum for video and discussion), you can double what you drip - like 30+ drops, and it lasts a LONG time. None of these methods take any time at all, seconds, really, once you get the hang of it, and there are videos in the V4L forum and I'm sure elsewhere on how to do it.
Not a great idea to drip into the batt end, really.
I refill/top cartomizers all the time, and mix flavors - I just topped a half-done chocolate V4L carto with some TV butterfinger, very yum, and it'll last me hours. And I chain vape big. I can't imagine dripping constantly, would drive me crazy.