Yes you can and all the what you have to do is :
1- copy all the mail boxes of LN 5.11 (R5) to the server of LN8.5
2-run the command of ” Load convert -r mail\( The LN5.11 mailboxes names) * mail85.ntf
3- Then you have to upgrade the ODS to ODS51 so run ” Load compact -c mail\( The LN5.11 mailboxes names)”
You have to run the above steps one by one for each mailbox.