Hi,
We asked the same question at the Epcot ticket window last year. I had four park-hopper passes, each with one unsed day, and wanted to buy only two annual passes. They allowed me apply the days from two of the park-hopper passes since I was buying two annual passes. The other two park hopper passes that I could not apply to my purchase will be accepted the next time I buy two more annual passes.
BrianBWV