Can these be used in a string I already own?

  90% of the time this will work. There are 2 points of contact the bulb needs to work; one on the bottom and one on the side. Most of the time our bulbs will work in other strings, so yes, I would try it and if they do not work then you may need to replace the string or try a different brand bulb. You will also need to make sure the string can handle the wattage of a 7-watt bulb in each socket.