Re-canning commercially or previously canned items is not recommended. While a few approved and tested recipes call for using a small amount of commercially canned tomato sauce or paste, it should be avoided otherwise for safety purposes.

But yes, all commercially canned tomato products should contain citric acid already - check the ingredients - so in theory you shouldn't have to add more if you choose to do it.

Dave

Dave is correct.
Using canned tomatoes is not recommended if fresh are called for. They will not be the same in density, as they are already cooked and more dense.
You should follow a safe, tested recipe for all homecanned foods. Using your own recipes is not an advised thing to do. You don't know the ph level of your recipe, nor the density. Both are important factors when canning.
Elizabeth Andress from NCFHFP, who wrote the USDA guidelines has advised us from using previously canned tomatoes in canning sauces, unless the recipe specifically calls for doing so.
Freezing your own sauce using canned tomatoes would be the safe way to go.

adding some vinegar or acid to lower the ph is always good. the lower the ph the better. I see nothing wrong with using pre canned toms but they get soft easy. the more heat the softer they get. they will basically turn to water or tom juice. good to eat but watery.

adding tom paste is good as it thickens the recipe.

if worried about ph just add some acid. lemon juice, lime juice, vinegar or citric acid are more common.

Citric acid is a much better way to add necessary acid to tomatoes. Vinegar tends give off flavor that may not be desired, unless it was for a salsa. Then again, cider vinegar or bottled lime juice can also be used in salsa. To get thick sauces, I like adding dried onions and peppers, and also use canned tomato paste, or tomato powder. My last big batch of tomatoes for sauce, were done in the Villaware food stainer, but I cut each tomato in half, and dig out most of the liquid and seeds before running them through. You end up with a nice thick sauce that way, with les cooking needed.