I don't think there is a single keystroke, but you can do it with ALT+I followed by enter (assuming that you have already positioned the cursor in the row below where you want to insert the new row.
It is almost as easy to just Rt-click the row number on the LHS which immediately opens a menu, then just click insert.
If you then wish to insert more rows, just press F4 as many times as necessary.
This is with Office XP (2002) and 2003. I have no idea what MS have done with Orifice 2007
SD