BTW they are cartomizer aka cartos if it is a 2-piece. Some 2-piece vendors still call them carts but here and on retail-price vendors cartridges (aka carts) are the mouthpiece and juice cup of a 3-piece.
Unfortunately some ripoff "name" brands don't sell blanks or juice and may try to make their cartomizers as hard to refill as possible. If you are lucky, the mouth-end caps are soft and can be popped out with a paper clip stuck in just enough to pop them out. If they are hard caps or worse, glued-on caps, then trying to pop them out may mangle the cap but you could stick one of these drip tips on the end as a mouthpiece instead. I do this anyway with many cartos. Just don't drip into them on a carto - the juice would probably go down the central air channel. Good as a mouthpiece though
510/901/KR808d-1 Delrin/Plastic Drip Tip
510/901/KR808d-1Transparent color Delrin/Plastic Drip Tip
If you can get the mouth-end cap out you should be able to add ejuice to the stuffing. First check for an inner seal (a donut-shaped piece of soft plastic inside over the stuffing - you would have to remove that. You should add juice while the carto is still damp with juice. If you run a carto dry you can singe the stuffing then you may as well throw it out. if you keep them damp you should be able to get several days use out of a good carto or longer.
if there are no inner seals blocking the path of juice, you might also be able to use the carto condom filling method, pushing juice into the carto. But that can be messy.
Note - if you have automatic (no button) batteries be very careful with refilling cartos - if your carto drips juice you could possible kill your battery since autos are dependent on a drag sensor in the threaded end.
I use KR808D-1 (aka Vapor4Life) with manual batteries. Greensmoke is its near twin KR808D-2 which is favored by ripoff rebranders - it has different threads and can't use any of the normal retail price KR808D-1 stuff without an adapter (LeCig sells D1 to D2 and D2 to D1 adapters). Vapor4Life's cartos are all soft cap. They also sell blanks and juice. You can also buy blank KR808D-1 soft cap blank cartomizers elsewhere if you have a KR808D-1.
There are videos of various methods on Youtube. search for
fill or refill
with: carto, cartomizer, Greensmoke, V4L, vapor4Life