Yes you can refill carts. I am assuming you are really talking about carts aka cartridges, the plastic and stuffing parts that push onto the atomizer of a 3-piece ecigarette
If you have the kind of cart that pushes on OVER the atomizer, then to fill you may need to push up the
juice cup with a paperclip leg stuck in the mouth end of the cart
If you have the kind of cart that pushes into a hollow-tube shaped atomizer, then you should be able to see the stuffing
Either of the above can also be washed (the stuffing and cart must be dried before reuse) or you can replace the stuffing
If you have a cart that screws onto the battery, then that is not a cart. That is a cartomizer aka carto and they are not filled the same way