I don't think you can do that.
What's the point anyway? Hitting enter already lands your cursor at the beginning of the next row. Just sort the rows when you're done or whatever.
Alternatively you could try pressing Ctrl + Shift + Right_Arrow when you're done with your inputs, followed by Ctrl + Plus to insert a line above. This is, however, not a very comfortable way of working with Sheets and doesn't work in Firefox, for example.