Yes, you can merge volumes by following the steps liste below:
You will notice that MailArchiva creates a new folder for each day in the active volume. The folder names are in (YYYYMMDD) date format (e.g "20070621").
(1) Find the volume which has the folder with the oldest (lowest) date.
(2) Open that folder and find the message that has the oldest (lowest) date volume (e.g. 20070621060739578.mrc).
(3) Record the name of that message (e.g. 20070621060739578).
(4) Repeat steps 1..3 above for the newest (highest) date.
(5) Create a new store directory for your new volume.
(6) Copy all messages from all the volumes in your new store directory.
(7) Create a volume info file in the root of your store directory. Set the modified date with the newest volume date and the created date with the older volume date:
# Archiva 1.3 Volume Information
# note: this file is crucial - do not delete it!
(8) Create a directory for the volume index.
(9) Add the volume to in your MailArchiva Configuration Screen.
(10) Click the Reindex button to create a new index for the combined volume.