Is it permissible to combine the intention to make up ramadan fasts & the first 9 days of dhul hijjah, hoping for the reward?
There are numerous opinions, I personally don't see it permissible to combine the intentions generally, but in this example the fact that you're busy fasting an obligatory fast on those blessed days which are of significant reward = is something to hope that Allah accepts your make up days with greater reward, hence your intention is that you're aware of the significance of the day but choosing to only intend to fast as obligatory = is better than what you aspire in combining intentions. One obligatory fast is more important than a hundred supererogatory fasts.