At present we only allow access to mail, calendar and contacts belonging to the authenticated user. So, your scenario of an admin accessing the conf. room calendar is not supported at this time. Access to resources belonging to other users e.g. conf. room calendar is on our roadmap to support but we don’t yet have a timeframe to share with you.
In the meantime, you have two options.
Option #1: If you are able to run the app on a server, you can build a service app that requires admin consent, but is authorized to access any mailbox in the Office 365 tenant. You will need to make your native app talk to the app running on the server. See this blog for more details.
Option #2: You can use our Exchange Web Services SOAP API to implement your scenario. See Office 365 shared calendars for more details on this option and relevant links.