If you want to add the same title to all the slides, then type it on the
Master Slide (use a text box). If the title is just on some slides use a text
box again (if the title space has already been used.) Is this what you want
to do?
Amanda,
Couldn't you apply the "only title" slide design. Select your slides,
activate the slide design task pane, click on the arrow next to the only
title design and choose apply to all.