If you eat a kezayis of bread you bentch. If you eat a seduah of pas haba bikisnin you also bentch.
If you ate slightly shy of a kezayis of bread and have only mezonos available, can you fill up the kezayis using mezonos or pas haba bikisnin food items?
r' moshe Feinstein in his responsa "iggros moshe" (exact source will be brought very soon! sorry!!) says that one may not, even for a rabbinic obligation, the reason being that the torah (and chazal) require a "ma'aseh achila" which only takes effect on a k'zayis, only once you have a ma'aseh achila of a k'zayis can other foods be used to complete a shiur of k'beitza and the rabbis ruled to recite bircas hamazon. in our case, again, there is nothing to be m'tzaref to, it isn't anything.
