For a deeper look into our Eikon Data API, look into:
Overview | Quickstart | Documentation | Downloads | Tutorials | Articles
Eikon Data API deals with content it allows you to request a set of data on a set of instruments and next you can process the content retrieved as is required per your needs.
Eikon Data API does not allow to upload portfolio holdings into Eikon, at is intended, at least at present time, solely for Eikon Data retrieval.
But I am answering from Eikon Data API perspective. There may be more information, such as on Eikon tools, that may be helpful in your task. I would recommend reaching out to Eikon support via Eikon->Help->Contact Us, and describing your requirement in full detail, to see if it is possible by other means, such as a third party tool.