Those daffodils must be very happy if they have naturalized or spread that much. The answer to adding perennials is to simply dig a large hole to add each new perennial to the bed. Remove all bulbs from the hole. The perennial will thrive and crowd out the bulbs in the immediate area especially if you water the perennials over the summer so that the roots spread out. Then you will have daffodils in the spring in the same area as perennials in the summer. Keep growing, Marianne Binetti