Many people use syringes to fill cartos. I agree with Rosco that the needles you have are probably not big enough to use with juice but if you have a big supply it wouldn't hurt to try it. Just remember what you used it for and don't use that one for your meds!
To fill a carto, you need to take off the end cap even if you are going to use a needle. You can stick the needle in the filler material or just drip juice into the filler. I drip juice into mine and have done so since I started vaping. I find it to be the easiest way to fill and I can see exactly how much juice is going in and when the carto is full.
The condom method is popular but I myself do not like it. I find that is wastes juice because some juice needs to be wiped off of the carto after filling. Juice gets in the threads and the battery end connection which can be a problem if you use automatic batteries. But, that is just my opinion. Try it and see if you like it.
You can add juice to cartos that came pre-filled. There is no problem doing that. A lot of people have bought pre-filled cartos and decided that they like that flavor. You can normally buy the same flavor juice to refill or you can try refilling with a similar or complimenting juice. The only thing that can be an issue is that some pre-filled cartos have hard caps that are glued on. If your cartos have soft caps, you are good to go.
