If we understand you correctly, the kippa fall you have in mind covers the top of the hair something like a wig, but blends in with a woman’s hair, which she leaves out underneath.
One can make a halachic argument to permit head-covering with this type of a fall so long as it covers most of the head and not too much hair is left out. (For many halachic authorities, this is not more than a tefach. For more discussion of how much, please see Head-Covering IV.) Using a kippa fall is better than leaving the head fully uncovered.
However, taking the rationales behind the mitzva into consideration, a kippa fall is less than ideal. On the one hand, it is hard to consider a kippa fall worn alone as a form of attire for the head. On the other hand, if we look at it instead as modest hair-covering, it is often indistinguishable from the hair like a wig, but (as typically worn) it lacks the key attribute that those who permit wigs generally emphasize: providing fuller coverage of the hair. We are not aware of halachic authorities who have explicitly endorsed using a kippa fall.