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Will we be able to add external components?

You cannot do that right now. There are a few libraries already bundled which you can use. But we are planning to work on adding this feature.

Or maybe if I create a custom component inside sketch, it would be nice if it's added as a template in the components toolbar.

That's a great idea! We need to add an accounts system before we can do that, but I'll add that to our list.