You can start with the cart that they are in.
Or you can IMPORT <filename>.PNG and that will import a sheet of sprites at Pico-8 command line. That will wipe out all the sprites in the currently loaded cart though!
Or you can SHIFT-drag on sprites to select a rectangle of them in cart, ctrl-c (or Command-C on Mac) to copy them, then load another cart, select a place in the spritesheet and paste to paste them there.
You can use RESPRITER (linked in the Jam page) to re-organize / swap sprites in the spritesheet.
Hope all that helps. Message if you need more help!