Certainly, customer due diligence and enhanced due diligence should be mentioned in your AML policy, but there is no need for a separate policy and I doubt that there are many potential donors among your readership. Consider enhancing your BSA/AML policy with the appropriate paragraphs and supporting them with appropriate procedures to be used at account opening and over the life of the account.
In essence, due diligence at account opening is about asking questions the law does not require you to ask, but which would help you to both predict the customer's future activity and identify customers subject to "expanded examination overview." Due diligence during the life of the account translates to reviewing standard reports in an attempt to identify unexpected behavior. In both cases, the actions are foreplay for enhanced due diligence wherever circumstances merit.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.