Datastream content is available for programmatic consumption through Datastream Web Service. Documentation is available following the link below.
However, to the best of my knowledge the license for this Web Service is not included with Eikon. To find out more about licensing requirements and what's included with your contract with Thomson Reuters, please contact your Thomson Reuters account manager.
Please see Alex Putkov's answer. Not yet all Datastream content is available in Eikon yet. What you could do though is to retrieve the RICs (Reuters Identifier Codes) for the instruments you are interested in through the Datastream Excel add-in. You can use these RICs with the Eikon "get_data" function. Hope this helps.