Sure, I'd measure it in a separate container though, one that you could keep the salt/water mixture for tomorrow as well, I calculated your tank and it's a 1.5 gallon critter keeper which is a bit easier. But right now you're probably not even using a fourth of it lol.
So measure out 16 cups which is 1 gallon, in a separate container/jug/jar/whatever and mix up the 2 teaspoons of salt in there. After it's mixed and you've added your water conditioner then you can pour some of it into her critter keeper and then put her back in.
However when you put her back in, please don't just plop her in, the salt will shock her system if you do! And I'm sure you don't want that lol
So to acclimate her (make sure the water is also the same temp as her current water) when she's in her cup with her old water, float that cup in the new water.
Every 10-15 minutes add in about 1 tablespoon of new salt water to her cup so she can slowly get used to the salt. You'll have to do this for a bout an hour for the first transition from fresh to salt water.
After you do this the first time and when you do it the second time you won't have to wait the whole hour because she'll be used to the salt water. But you still need to do that for about 20-30 minutes and because she is old, use precaution otherwise bad things might happen. Also if you can get a heater in there with her and set it to 80 that would be best. They are tropical fish and need tropical temperatures and just like older humans, old fish need special care too. So a steady temperature will do wonders for her :)
And you could probably fill the keeper a little less than halfway so half a gallon then, for her. If you have, you can put some fake plants in there with her so she can sit close to the surface to breathe when she needs to. It will be a lot less stressful on her then.