First of all, the fact you have 1000+ Access Rules suggests a cleanup exercise of your rules is probably in order.
If you want to code something up with the API/mgmt_cli, you can:
- Iterate through each rule and look for the relevant object in Source/Destination.
- If you find said object in the rule, use the API to modify this rule to include the new object where the other object is.
A potentially easier way to achieve this is to put the existing object and your new object into a group and use the the "Where Used" functionality.
You can use that to replace the existing object with the group that contains your existing and new object.