I guess there's no way to insert an empty line or column in HP Prime Spredsheet App.
The way I solve this demand is described below. Assuming that one extra line is to be inserted at line 4:
1) Place the cursor at the first column of line 4:
2) Touch the softbutton "Select" on the screen itself, just below column "C" in the picture. Then this button becomes "Sel." (don't press it yet). You can now move the cursor right and downwards so that you highlight all the lower portion of your table.
3) Press "Copy" (Shift+View). Then press "Sel." softbutton.
4) Move the cursor to the first column at line 5 and press "Paste" (Shift+Menu). Select the array of numbers and press Enter. Then select "Values" if that's what you want:
5) Press ENTER and you'll have it pasted from line 5 and down:
6) Notice that your 4th line is still there. Now you can overright it.