This is an old post, but in flex 2.0 you should checkout this link
In order to use Twilio Paste inside your plugin, please use Flex.setProviders() as follows which will wrap the Flex UI with the passed Paste theme provider. This will allow you to use Paste elements and design tokens within your plugin. Ensure this is done before declaring any components.
import { CustomizationProvider } from "@twilio-paste/core/customization";
import { Button as PasteButton } from "@twilio-paste/core/button";
Flex.setProviders({
PasteThemeProvider: CustomizationProvider
});
Flex.AgentDesktopView.Panel1.Content.add(
<div key="A">
<PasteButton key="A" variant="primary">
Paste Button
</PasteButton>
</div>
);