Let's say you have a text you want to animate. Save every frame of the animation as a seperate BMP file with the same name each time, the only difference being a +0, +1, +2, etc. in front of the filename. For example:
+0footext.bmp
+1footext.bmp
+2footext.bmp
+3footext.bmp
And so on. Then, in Hammer, texture the face you want to animate with the first frame (+0), and there you go. You can only have up to 10 frames.