Due Date in FM derivation rule

Hi Experts,

I need to use Document Due date as a source field in deriving the right FM objects. I am unable to find it in list of fields available in defining derivation rules in FMDERIVE. I do have document and posting date in FMDERIVE. Can any one suggest how i can achieve this in defining derivation rules. my requirement is something like bellow

Document Due Date + Cost Element + Cost Center = Fund + Fund Center + Commitment Item

I need to use document due date as source field in most of my scenarios.

Regards,

Atif Amin