I don't think we have support for expanding variables in the plugin currently. This need to be implemented on a broader spectrum in the code for the plugin.
Which input field did you try with? I will add it to our internal wish-list for new features to the plugin but can't promise anything.
There seems to be doumentation for implementation of this feature on Grafana.com. It is also possible to implement and make a PR on GitHub for any developer since the plugin is open source.