Rewire a string of lights using 14/3 or can I add a red wire?
With traditional switches

You are using the word "traveler" in singular.  3-way switches do not work that way.  They need 2 "travelers".  

So if you are currently supplying hot and neutral down that /2 cable, you cannot simply add "a traveler" to the pair.  You need to add 2 travelers, so you need /4 cable, which is expensive, or in some cases, /5 cable, which is unobtanium.  

So you are actually better off leaving the /2 cable in place doing whatever it's doing... and adding a /3 cable between the two switch locations.  This will keep the wiring simple.  

With smart switches

Smart switches either need 1 traveler, or 0 travelers, depending on their particular design.  With the right smart switch, you may not need to do any modifications to the wiring.  Shop carefully and save yourself some trouble!