I think copy pasting is not possible. What we did is provided a template which the person could upload and process. This way I can make sure that validations are done on the data.
Some creative ideas, probably not suited for your situation, but here goes nothing:
Create a macro in excel that transforms selected cell contents to csv and puts it on the clipboard.
Then let the user paste it in a text area in Mendix and parse it.
You could even have the macro submit the csv content to a published rest service automatically.
Personally, i’d go for uploading the excel file as Ronald mentioned.