This can only be done after acquiring the green keycard. Start by heading to the teleporter that is placed before level C-1 . Then, enter that level and proceed until you find a green door, which you should enter. Use the human form of your character to go under the gap and then get the 3 items on the right, which include 1 life. Now, go back to the teleporter (which resets the position of all items in the levels) and repeat the step above, something you can do until you have up to 9 lifes, the maximum value for them in this game.