Is there a way to copy or spread a Cube View combo box selection across cells?

You cannot directly copy a combo box selection and paste within a cube view. And if you use a CV in a dashboard, this does not work either. However, the following should work:

Embed your CV into a spreadsheet
Save the spreadsheet in the Workspace
Create a Spreadsheet component in a dashboard
Link the Spreadsheet component to the Workspace dashboard which itself pulls in a CV
Put the spreadsheet component into a dashboard

If you do this, you can right-click on one cell and hit copy, then multi-select all other cells and hit paste.