Grow a degraded raid6?

Any chance of being able to grow a degraded raid6 array?
(missing one drive) I tried to add a spare without
it immediately grabbing it and resyncing but couldn't.

Why? I made the array raid6 because eventually it'll have
8-10 drives. Currently you can't change from raid5 to 6 and
preserve your data. Now it has 4 drives and I'm happy with one
redundant for that small number of drives and the performance
isn't a problem.

Thanks!