I'am sorry for not getting what you mean,do you mean creating a new rule by copying existing rule ? A rule is just a node ,you can create a new rule by using node copy api, or you can first get a rule and modify id then save the modified rule .
If I understand correctly, You have this requirement
For an example:
1)You have Folder A - Create a custom rule[RuleA] for all cm:folder under Folder A. 2)And this RuleA will create another rule [RuleB] on ChildFolder/sub folders
Yes, you can do this. In your custom RuleA's ActionExecuter you can create another rule [RuleB] using RuleService and can apply/save this rule on subFolder.