OK, never mind. I believe the signatures are encrypted, so you can't export or import them digitally. Frankly, I don't understand why they're encrypted. You have to log in as me to even get a jpeg from my system. If my signature were a jpeg owned by me, you wouldn't be able to get it if you weren't logged in as me.
The Signature app on Preview would be handy for emplacing more than signatures. If I could emplace any jpeg with Signature, I could use it to "stamp" a pdf document with some logo. I don't believe Preview offers any other way of doing that.