Hello - I received a promotion flyer from Marriott today about earning 2000-5000 points when I book with Hertz using the Marriott's CDP #.

For all my business travels, I book through AMEX's company travel portal and there's no place to enter additional CDP #. So I was wondering if I can call the CS and tell them to add on this Marriott CDP # to my existing reservation. Is this considered double-dipping? Thanks.