Can a Drop Down Box be added to Advanced Guestbook 2.3.1

I'm using Advance Guestbook 2.3.1 (with 2 separate databases) at my fanfic site - one as a regular guestbook and one for reviews of the stories.

I would like to add a drop down box to the page where the submitter is leaving their comments so he/she can let me know what story they are reviewing. Can this be done? If so, easily?

I'm not sure what files I would have to modify to achieve this.

I've searched high and low for "Reviewbook" scripts but I haven't found anything that meets my needs for allowing a person to review a story online. The best I've found is modifying the AG 2.3.1 and adding either a title line or, preferrably, the drop down box, would make it perfect.

Any suggestions would be appreciated. Thanks.