Pegasus Mail & Mercury

How to put mails from 5 different computers together?  I used Pegasus since forever. And, whenever I moved, I simply made a backup of my emails and started afresh on a new machine. In years, I got backups from say, ''1998-01'', and ''2004 only'' and ''2001-2005, without 2004''.  I need single copies of all mail stored in all these ...