During a transaction you can insert a blank above any selected item. Once you have an item selected in your transaction window, bring up the miscellaneous tab by pressing F12 or using the Miscellaneous button on the bottom of the screen. Then to insert the blank line, press Shift-F8, or use the Insert Blank Line button on the right hand menu.
Note that the blank line will be inserted above the selected item. You can insert a new item into the space by selecting the empty slot and then entering an new SKU into the transaction as normal.