You have a couple of options:
1 being the built-in post function - "Create a sub-task"
Of course you can run a script to check for desired conditions. Say for example same workflow is used for different issue types, you could just specify to run for "Epic" issue type.
Another option might be running a script listener, since you mentioned that you don't want to change the workflow.
Insert a new script listener, which runs only for specific projects and also you can specify later in the script issue type condition.
You would to write your own script here, where in the 1st option is much more convenient.
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