Can I use an old cog for my new hub?
To quote the page you linked, "Since the Velosteel hub has standard track threads, it can run any fixie/track cog built to Phil Wood/Campagnolo standards." For cogs, this is the same standard as every other cog, but the lock ring will probably be different. Just take the cog from your old hub and keep the lock ring from new one.

Note that this is different from most other coaster brake hubs, the more common Shimano, Torpedo and others use splined cogs.