I'm reasonably sure that the 'sustain a spell' effect only works on D: Concentration spells.
If you have a D: Diameter spell that you want to use as a D: Sun sun spell, modify it using MuVi.
This has been debated somewhat though.
However, you can design a 'sustain spell' effect with a duration other than D: Diameter, easily enough.
EDIT:
Ah, the thread I was looking for was here