Hi,
I don't think this will work. Instant-On products are different from Instant or Unified Family of products..You might want to consider AP 305 if you really need to extend your existing cluster.
However, just note that IAP 205 will only support Instant 6.5.X code so you might want to consider building a new cluster based on newer APs which can support newer code like 8.X. In this case, you can consider AP 505 or other models...