It can be done lah.
On folk guitars with metal strings, the tension of the strings would had been counter-act by the truss rod.
But if you place low tension nylon strings, you only need to set back the truss rod to set the correct action on the frets
Don't fret about it if you don't know what is truss rod and how to adjust, its very simple. Youtube plenty of examples.
BUT, a classical guitar with Nylon strings cannot have metal strings because classical guitar has NO TRUSS ROD.