Back on GP 2013 we added the ability to Copy and Paste from Excel functionality in the GL Transaction Entry Window, this was a huge request by our customers. Right now this does not work on web client, only rich client.
The stumbling block you run into is the copying the excel data from the client machine, the server cannot access the client clipboard to paste in GP in web client. Would be nice to have this in web client.
Category: Web Client
Under Review
Ideas Administrator

Thank you for your suggestion. We will consider this functionality for a future release.  

Best regards.