Combining Qadha fasts with fasts of Zul Hijjah
In the name of Allah, the most Beneficent, the most Merciful.
Saaiduna Abu Hurairah Radiallahu Anhu narrates that the Prophet of Allah Sallallahu Alahi Wasalam said, ‘On no days is the worship of Allah desired more than in the first ten days of Zul Hijjah. The fast of each of these days is equal to the fast of a whole year and the worship of each of these nights is equal to the worship of Lailatul Qadr’. (Sunan Tirmizi p.158 v.1)
Saaidah Hafsah Radiallahu Anha reports that the Prophet of Allah Sallallahu Alahi Wasalam used to fast the (first) nine days of Zul Hijjah. (Sunan Abi Dawud p.338 v.1)
With regards to your question, it is preferable that during the first nine days of Zul Hijjah one keeps the fast with the intention of Zul Hijjah and keeps the qadha fasts later as some scholars are of the view that one does not receive the reward of optional fasting when combining with qadha fasts. (Fatawa Darul Uloom Deoband p.495 v.6)
However, if a person fasts during these first nine days of Zul Hijjah with a primary intention of qadha and makes intention of Zul Hijjah or Arafah as a secondary intention, then hopefully by the mercy of Allah (SWA), both intentions will be accepted.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.