https://learn.microsoft.com/en-us/answers/questions/1197913/front-door-waf-policy-hierarchical-rules
based on a similar question, it seems there currently isn't a way to implement hierarchical WAF policies on Front Door or to have multiple WAF policies for a single Front Door endpoint. A single endpoint can only have one WAF policy at a time, and WAF policies cannot be assigned to the entire Front Door, only to individual endpoints. Furthermore, the policies in Azure Front Door and Azure Application Gateway are distinct from each other and cannot be used interchangeably.
However, as s an alternative where you can have a predefined template of WAF policy with all the required rules, modify it according to the endpoint(s) you are going to assign, and then create and apply the WAF policy to the respective endpoint.