Probably a bad idea.
Water from a freshwater tank has tons of happy little bacteria in it, when you add the salt to it those bacteria will die causing an ammonia spike.
This'll probably cause your tank to re-cycle itself depending on how much water you change out - and a bacterial bloom (cloudiness.)
Just grab some Distilled water (by reverse osmosis) from wal mart for like .41/gallon.
Whenever I start out a tank i just use dechlorinated tap for the bulk of it and R/O water for the water changes. Never had any problems with that.
Basically, get new water - don't use the stuff from your freshwater tank h*