Can I re-can a batch of salsa that is missing an ingredient?
The conservative answer is that you should only do canning based on an approved, known to be safe recipe from a reputable source such as a university extension or a safety agency.

Having canned the salsa without the sugar, you have deviated from such a trusted recipe, and so the contents of the jars has to be considered at risk, as the sugar may have been a part of the reason the contents was safe or required less processing time than it otherwise might have (sugar in solution reduces the biological availability of the water).

This is especially true if the salsa was not made to have a pH of less than 4.6, to qualify as a an acid food.

Therefore, you should not adjust and re-process as the major ingredient may contain toxin.

Please see the information at the National Center for Home Food Preservation.