You can add individual risks one at a time to the list editor or you can paste in a whole list of risks from another document.
Copy your list and then go to the List Item Editor in Matrix and use Ctrl+V to paste the list.
If you have short names or risk IDs as well as the risk descriptions use Excel to create a list where the short names are in the first column and the risk descriptions are in the next. Copy both columns together before pasting and the information will be pasted into the correct fields in Matrix.