Ooooops! I read that as adding text to a slide, not adding a slide of text. Is the latter what you meant? You can do that. There are several ways one of which is:
1 - in your text editor of choice create a custom, landscape page of 8" x 6" (for the 4:3 ratio used in iPhoto's slideshows).
2 - add your text to the document, format and color as desired.
3 - Do a Print to PDF but select the "Send PDF to iPhoto" option.
4 - you'll be asked to select an existing album or create a new one to add the jpg file of the document to.
This will create a fairly high resolution jpg of the document you can use in the slideshow, books, etc. Here are two rough examples of a letter sized document in both
portrait and
landscape orientation that I put in a book for demonstration. Once in iPhoto you can crop it further if necessary.