Hello,
Adding to Marco's reply...
I opened my NAV 2016 database and tested your scenario. Mine tested corrected. Here are my steps:
Create Header of Purchase Order:

Create Excel Spreadsheet with columns in order:

Add Type Items to my Excel spreadsheet:

Highlight just the columns and rows I want to copy:

Go back to NAV and in the Line Fast Tab, using my mouse, making sure my first line is blank, point the mouse to the box next to Type column and right-click to show list and choose Paste Rows:

Spreadsheet rows pasted and the Items description populated in the description column automatically:

Hope this helps.
Thanks,
Steve