You can also top off with saltwater and let it evaporate. Brings it up nice and slow.
That's how I'd do it. I have just added some normally mixed saltwater to the sump above the normal water level. This gives the water a slight bump in salinity. Then as it evaporates, the salinity will continue to rise. Just check it daily and adjust as necessary in small steps. It will take a couple days and you will be right back where you want to be.