How to add PTR DNS record for my EIP without AWS-hosted Forward zone?

To create a reverse DNS (PTR) record, AWS must be able to verify that there already exists a forward (A) record that maps the domain name (e.g., host.example.com) to the Elastic IP address you've allocated.

It doesn't matter whether you are using Route 53 to serve the domain or not. You can use a third-party DNS service to host your domain and it will still work. All that matters is that AWS can validate that the A record already exists.