Massimo, do you really want to restore an item? As far as I know a "restore" in Retain creates only a stub but not a mail itself.

I would not use Retain. Like Diethmar said, it is using GroupWise Stubbing which it not optimal at all (its not the real orginal message being put back into the live on-line GroupWise mailbox).  The Retain Stubbing would be my second choice over using a PO Restore Area method as my primary choice.

I would use the GroupWise built-in Message Restore solution using a "PO Restore Area" or use the GroupWise Disaster Recovery (GWDR) software solution (which uses the PO Restore Area).  But, GWDR would have had to be up and running prior to the data loss incident that you are in today.

I'm not dealing with a backup situation here. The messages in Retain originally come from a different system (Exchange on Prem).

Maybe you ask some Retain supporters/devs in the background. There was a migration tool based on Retain to migrate from Exchange to GroupWise. However official support for this activity has been stopped for some reasons. I am sure you have good connections ... 

Today, Retain 4.11.0.1 can export to PST file format where you can then Import that into GroupWise using Transcend or *maybe* the very old "GroupWise 7 Migration Utility SP2 for Microsoft Exchnage" if it can be found somewhere.

Many years ago the GroupWise product had its own Exchange to GroupWise PST Import Utility called the "GroupWise 7 Migration Utility SP2 for Microsoft Exchange".

Here is a TID on it - https://support.microfocus.com/kb/doc.php?id=7015480

Here is the on-line documentation for it - https://www.novell.com/documentation/gwutilities/gw7_exchimp/data/gw7_exchimp.html

BUT, this is a very old solution from the GroupWise v7.x days.  You can run the GW v7 Windows client against a GW 18 back-end, its unsupported but it could work.  The v7 VEW files would also need to be copied to the PO file structure if you want full functionality.

Micro Focus since then has partnered with Transcend ( https://transend.com ) to provide an Exchange to GroupWise migration solution. Their tool is named the "Transcend Migration Console".  It does PST to GroupWise migrations PLUS MUCH MORE.