"Θα μπορούσα να είχε ρωτήσει αυτό."
Translation:I could have asked this.
Sometimes this structure is interchangeable (and indeed «το» makes it more colloquial, we use it a lot) but sometimes there is a different emphasis and meaning:
Θα μπορούσα να το είχα ρωτήσει αυτό = I could have ASKED this (but I didn't; I missed my chance, and maybe I didn't ask anything at all)
Θα μπορούσα να είχα ρωτήσει αυτό = Ι could have asked THIS (THIS instead of something else that I asked)
Intonation is also important! Remember to stress the right word each time.
One extra sentence: Θα μπορούσα να το είχα ρωτήσει εγώ αυτό = Ι (I am the person who) could have asked this, but someone else did. (In this case, you can't possibly omit «το». And you must stress the word «εγώ».)