Apologies if I've misunderstood what you are wanting to do, but I've put multiple lines of text in numerous cells just by keeping on typing. The text adjusts to the size of the cell and automatically takes a new line when you run out of space on that line.
If you want a new line but don't want to fill the line you are on or decrease the width of the cell, start by typing your first line of text, then press the space bar a few times then type the word you want to start your next line. Keep adding spaces until that word drops down to the next line.