How to preserve curry paste
Rather than changing your recipe by adding preservatives, there a number of approaches you could use, depending on the acidity of your curry paste. At or above 4.6 pH would have to be pressure canned to avoid botulism, as it takes the temperature to 240°F / 116 C (a water bath heats to 212°F / 100 C).

The simplest way is to freeze the paste. Secondly, for an amount you're likely to use over the course of several months, it could be stored in the refrigerator. Heat the paste with a bit of oil and of water. Completely fill sterilized jars with the hot paste, close, and turn them upside down to cool completely before refrigerating. 

If it is high acid, it can be water-bath canned: fill sterilized jars with the paste, top with sterilized lids and bands, and process the jars in boiling water. After cooling, check to ensure the lid is sealed; if not, it can be re-processed.