I only know how to do this in PS 7, but you should be able to do it in other versions.

1) Open up a picture that you'd like to add a stretch to in Photoshop.

2) Using the Single Row Marquee Tool, click on the place where you want your line stretch to begin.

3) On your keyboard, press ctrl T.

4) Click the line, and drag it down as far as you want the line to stretch.

5) Go to close out of your image by clicking the X. When a popup comes up, click apply. The next popup that comes up, click cancel.

6) You now have a line stretch, yay! :D

I'm sorry if I didn't explain this very well, it's easier to explain if you have images to show along with it.