You could use a syringe, but its really not necessary. Cartridges hold very little juice so its just as easy to just drip in a few drops at a time.
What PV are you using? I dont know if you want to try cartridges just out of curiousity, but keep in mind that cartridges are generally a lot less consistant than cartos. Also cartridges hold a lot less juice, so you have to refill a lot more often. And cartridges can randomly dump half their juice into your mouth, and its more likely to happen if you overfill, so try to make sure you dont overfill them