I was on the same boat once - I booked award nights and wanted to add a few cash nights. My BRG was shot down because Marriott said it couldn't be combined with point nights (they can't calculate the value of it - they actually went ahead and look at what it costs at the OTA for all 5 nights even though I only wanted 3 paid nights). So, I cancelled the award, filed the BRG and then book the award after it was approved.