HELLO ATUL SHARMA..
In my opinion you can easily replace a power combiner with a circulator but i think there maybe difference in results though because using a circulator does affect your results…
Normally i have seen the circulator for adding and dropping wavelengths and in that process the results do get affected and moreover it you check the power levels after the circulator you will find the power level drops just in case of the WDM Mux..Still you can cross check by using it in place of power combiner and check for the results..
Thanks & regards