No, no, no! Don't do it! An instrument designed for nylon strings cannot handle the increased tension of steel strings, which is typically 3x that of nylon. The extra tension can make the neck bow, pull off the bridge and even cause the top to implode in the process.
Now, some sets of nylon uke strings can include a metal wound string or two, but the metal is wound over a nylon or filament core and it is not actually considered a steel string.
There's been a lot of discussion of different uke strings here, but in the end it all boils down to personal preference. Some brands/types of strings just seem to sound better on certain ukes than others.