Yes, you can but I don't think the solutions are stable so you would need to mix just before use. This is definitely recommended for Campden tablets which are a bit difficult to dissolve.
The amount of bisulfite used does not have to be dosed in exactly. If you can weigh out a gram you could easily estimate visually how much powder it takes to make up half a gram. You could also weigh out a gram, divide the powder into 4 equal portions, add the first portion and then check for chlorine smell. If there is no chlorine smell you've used enough. Note that the 1 tablet (~500 mg) dose is based on 3 mg/L chlorine equivalent and most suppliers don't use that much so that less than 1 tablet will often do. The reaction is fast and once the chlorine odor is gone you have added enough. Obviously if the chlorine odor persists you can add another quarter (or maybe half of a quarter) and check again. If you overshoot and add to much there is still no problem as metabite is harmless to beer (and people with the exception of those who are allergic to it).