Why? I don't think that's a good idea. Read the instructions on the medication. I'm sure you will not find any information about using it with salt. Use one method or the other, and stick with it.
As far as i know both will slightly lower oxygen levels in your tank and have
never heard of both used together,..lower temp of arround 75 and plenty of surface
agitation would help, but my advice is go with Melafix per the directions.
I prefer to use salt only. No Melafix and admittedly, I don't like it. Dose a teaspoon of salt per gallon. Repeat the last dosage after the next 12 hours. Any sodium chloride will do as long as it does not contain yellow prussiate. Whether it is iodized or not, is not the issue.
Both will indeed lower the oxygen level so be sure to aerate water vigorously. And salt does help prevent further secondary infections since it prevents bacteria from entering the open wounds up to the bloodstream.
Common sense suggests that Melafix would lower oxygen levels, but in fact it doesn't, so this is one thing there's no need to worry about. I can't imagine the amount of salt involved would have a huge effect either, but improved aeration never hurts anyway.