If allowed by your companies policy, you can ask for 2 things at the admins
1 - export archive to pst or
2 - export archive back to mailbox
1 - you need the pst + make a pst from your current unarchived mails still in the inbox. or
2 - you need to make a pst of the mailbox
for making pst of mailbox, if you are allowed, otherwise you need help from admin.
Option 3 (only if your number of archived items is not too big)
Open Search Safe (should be in Outlook an icon top right somewhere) search items, restore your self. Only possibleif allowed by admins
Or use Archive Explorer to do so
Regards. Gertjan