Delectatio

I am quiet new to Marriott (we are usually 'Hilton-people') but my partner and I did partake in the Marriott promotion for new Rewards members (1 free night 1-5 cat. for every 2 stays) so we have now earned 2 free night certificates that we want to use somewhere nice.
Some of the hotels where we can use the certificates have a club lounge so my question is:
Would a hotel allow us to pay extra on arrival (or during the booking process??) for club access (or a club room upgrade) when we use the certificates?
Anyone knows what the typical price difference would be (at Hilton there usually is a price difference of about EUR50 between an basic room and an executive room)?
Thanks a lot in advance