In order to run AWS monitoring scripts (http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/mon-scripts-perl.html) on one of my ec2 existing instance, I need a IAM
Earlier there was no solution. You had to create and AMI and launch the server again with the appropriate role.
On Feb 9, AWS launched these new CLI options which can help you solve your problem. See this link: https://aws.amazon.com/blogs/security/new-attach-an-aws-iam-role-to-an-existing-amazon-ec2-instance-by-using-the-aws-cli/