Can we use a multimeter to be an alternate for salinometer?
No, unless you built it yourself (assuming the nationals committee releases the same clarification as water quality). Even if you were allowed to use a multimeter, I would seriously dissuade it, because I don't know how thick the leads are, and they may corrode with usage and contaminate the solution.
You could build your own multimeter, but many aspects of electrical salinometers are difficult to work with, and potentially even dangerous. To start, you might consider measuring voltage across two jumper wires. But jumper wires are usually copper, and then you have to deal with your wires corroding. Okay, so you might choose a low-corrosion material like stainless steel. But stainless steel releases chromium when it corrodes, which is dangerous to your health. Okay, so you might choose a low-corrosion material without chromium, like gold. But gold is expensive, and it may not even work (but pretty much nobody on the forums actually knows because we've never tried).
As you can see, bringing your own multimeter for salinity testing can ensnare you with more troubles than anticipated. If your team has people good with chemistry, you could consider making one, but make sure to build one at the same time
as you build a straw one, because an electrical one will fail unexpectedly before competition, or the calibration will suddenly go awry, or it won't be done as fast as you think. It'll take a strong mind to not give up, but if your team is dedicated, I hope you succeed!