According to Dr Hessayons Flower Expert book you can cut and dry them for winter decoration. He suggests dividing clumps in April or May to propagate. You will probably get better answers if you join up and let people know which part of the world you live in. In UK he only recommends them as hardy in the south. I have only recently joined and am glad it will soon be winter as I will be able to browse the site more thoroughly.

If you have let a head go to seed, like I have you could try planting the seeds but weekens the bulb for next years growth. If its just the head of the flower that has died off no, you could do what Madaboutplants has suggested. In order to have more you have to divide as Madaboutplants has, as above

i have agapanthus in a large pot out side all year round and it is hardy in east yorkshire.

as morgana says you could try growing the seeds but they take several years before they are big enough to flower. I tend to leave the seed heads on for the dirds and then the frosts make it pretty all over again.