The easiest way to add text is to do it in post processing after you have exported the image from Poser. Load the image in a Piaint program such as PaintShop Pro, or Photo Shop, create a new layer, add the text to the new layer, when you are happy with the way it looks, merge the layers.

You can have text in front of your characters in Poser by using a transparency map on on a Poser square primitive. I don't have time to go into detail at the moment. Perhaps someone else can jump in here and provide more information on transparency maps.