Font Creator from www.high-logic.com allows you to management a series of signatures in a mail merge.  Using Font Creator, you can import a series of signature images into the software and assign each signature to a font character.

For example, launch the program, create a font (name the font anything that has meaning for you, such as "Central Region," then import each signature into a single character, say Mark Rodrigues' signiture is imported into the letter "R." 

In the data file, whenever Mark's signature is needed, use the letter R as the merge value. In the document, the line in which the signature should appear is formatted for the Font your created, in this case, "Central Region."  Instead of "R" you see the imaged signature.