Can I put steel strings into a nylon string classical guitar?

No. Steel strings put much more tension and pressure on the guitar than nylon strings, and classical acoustics are not designed for it.

Also, nylon strings sound excellent. Why would you want steel?
 
You will fuck up the guitar.
I like the sound of nylons, but not the feel. 2 different sounds, though.
I have a classical guitar; I would love to also have a decent acoustic, maybe a dreadnaught shape or one of those Martins like my friend has. Dunno which I prefer, though.
 
imho, steel stings make not that big difference in tension. in case you have normal acoustic guitar, not a kind of with too thin neck, steel strings won't damage the instrument. mine acoustic is already over 15 years old and I always used steel stings on it. nothing wrong happened since, I must note. but yes - steel strings have a way different sound.
 
It's not a matter of the neck being able to handle it, it's the bracing for the soundboard. The design of a classical guitar's bracing is not meant to handle the added tension of steel strings. If you want to put steel strings on a nylon string guitar, go ahead; but, you do so at your own risk and when the soundboard rips off because of the added tension, don't come to me like this: :cry:
 
simply buy normal guitars, not cheap crap that breaks from some extra tension you made with too loud chords :D or better remove all strings and store your guitar in a special case - so it will live forever.
if during 20 years of experience of playing guitars I have never seen any acoustic broken from steel strings that makes sense to me, at least. everyone to his taste, though.
 
My first guitar (very cheap/crappy) was made for nylon strings, but I put steel strings on it instead. It didn't sound any crappier than it did before.