Yes, this is possible.
There are two ways to do this:
1. Using a Boge soft cap carto, pop the end off - the soft rubber white plug in the top end -and insert a drip tip. A 510 drip tip is OK but some vendors who specialise in KR8s also sell drip tips for this purpose (like V4L). A Boge soft cap KR8 carto is exactly the same ID (internal diameter) as a stock 510 atty.
You can drip any one of a number of ways. For example, down through the drip tip. If you drip in the centre of the open carto it may leak or block the air hole, but dripping down a drip tip, especially if slightly angled, it just runs down the side and into the wadding successfully.
Or you can pull out the tip and drip into the carto, held at an angle.
The best liquid for this purpose is less viscous - thinner and more runny. Thick VG is not so good, but it's easily thinned with water, alcohol or pure PG.
Over-dripping will cause a leak.
2. You can also de-wad the carto if you are very careful. A Boge carto has an inner sleeve of heat-proof material wound around the vertical element, then thin sheets of batting/wadding wound around that, in a Swiss roll effect (like the rolled-up cake).
What you must do is to carefully find the outer edge of the wadding - using tweezers, a needle, or a straightened-out paperclip. Tease the edge up. Spin the carto between the fingers and thumb of the left hand while teasing out the layers of wadding with the pin or whatever in the right hand (or vice-versa). When the wadding has been extracted, the inner wrap will be slightly displaced - push it back down, carefully.
The carto is now mainly empty and will take thick liquids better. The drip is faster to hit the element. However it's much easier to flood it and cause leaks by over-dripping. You need a sealed manual battery for this. Some manuals still have a tiny hole as the same case is used for both manual and auto - you should plug the hole with some super glue or epoxy glue.
Drip with the carto angled, down the side wall. It doesn't matter if you use the drip tip in or not.
You can maybe try method #2 if the carto is plugged up after a number of refills. If you break it, then not much has been lost. If you succeed then it should work OK again. It's now basically a dripper atty so you can't drip any more than you would in an atty - say 3 drops or so. Any more and it leaks.
