Actually, a .028 inch nylon classical guitar string MIGHT be okay until you buy the right string, but before you use that guitar string, what harp do you have and what string broke? Does the envelope you got your string from give you any clues as to the size of the string?
You do have to be careful about the ball though. It could get stuck in the eyelet and that would not be at all good. I’d probably not use the ball end for that reason.
I hope this will be helpful. This is how I tie the 3rd octave strings on up:
Before you tie the knot, place the string through the soundboard. I don’t cut the length till a few days later. The clothespin idea is a good one, especially if the string is on the short side to begin with. Tie an overhand knot:
and before closing up the knot, stick your string tie into it. Tighten knot.
Then do a couple of half hitches with the string end closer to the board: (The technique is correct even though it’s a spinning demo)
Tighten your knot, pull the string snug against the soundboard, thread the whole length of string through the tuning pin, secure it, and bring it up to pitch.
Reward yourself with chocolate and wine.