Workflow transition before/after specfic date based on multiple metadata

Hi,

Currently you can use the transition 'before/after a specific date'. This 'date' is a document property but the number of days/months/week is fixed. An extension of this functionality where we would be able to also use a metadata for this 'number' would be great.

Example:
We have a procedure library and use versioning through a workflow to update and maintain the prodedures. The publication date is put in a medata (=date when document entered state 'Published') and we have a field where users indicate the amount of years after which the procedure will expire.
It would be great is we could use the following workflow transition:

Transition to: [Update] if "before/after a specific date" [Number] "years" "after" [Publication date]