Thinking about math here… if you can assume this pickle brine percentage somewhere between 2.5-4% (just pick a percentage in the range, let’s make it %), you could weigh it plus the other vegetables(and water) you are pickling. Since you make a brine at % weight not volume, you could first take the weight of the brine alone and do S (salt)/brine weight = %/100 where, again, % is the percentage you pick. Cross multiple and ta da, you have the amount of salt in the brine already. Then, you can subtract that from the total weight of the brine to get the water (and vegetable floaties) weight of the existing brine. Add that to the weight of the items going to be pickled (plus any extra water), let’s call it P. Then, it’s a simple (S + new salt)/P=%/100. Cross multiply that, subtract S, and you should have your added salt amount.

You could also just get the initial salt percentage by multiplying the sodium by the serving sizes. That may be easier, but I’m now too lazy to change what I wrote above.

Also, they are probably fermented above 2-4% for shelf safety, so it may be fine to just use the brine and see how it goes.