The only problem with putting ball-end "folk" nylon strings on a steel-string guitar is that the steel-string guitar is braced under the top to resonate properly with steel strings. The nylon strings only put about 40% as much tension on the top of the guitar as steel strings. So if the guitar in question has a thick top that is heavily braced, the nylon strings will not be able to "excite" the top to vibrate properly, and the sound will be a bit quiet and muted. If the top of the steel-string guitar is thin and lightly-braced, it will work better.
A rule of thumb is that inexpensive steel-string guitars are "over-built" with a heavier top and heavier bracing, and more expensive steel-string guitars are "lightly built" with thinner top and lighter bracing. More expensive guitars are more resonant, but also more delicate and less durable.
Also, the style of the steel-string guitar matters as well. Nylon strings would probably not sound great on a big dreadnought or a jumbo guitar, but would sound better on a grand auditorium, concert, "O" or "OO"-style guitar, which are smaller guitars that are braced more lightly as a rule.
Here is a link to D'Addario "folk" ball-end nylon strings.