If you have multiple archive files or .pst files you want to merge the steps below should help you resolve that issue. Before going any further creating backups is always a good idea in the event anything happens!
- Open both Archive/.pst files in Outlook, if you are not sure how to do that click here for instructions on that
- Expand both archives on the left pane of Outlook
- Go to the inbox of the archive/.pst you want to merge from and select all your emails from the inbox by pressing Ctrl+A
- Drag and drop the emails from that archive/.pst into the archive/.pst file you want to keep
- Repeat steps 3 and 4 for each folder in the old archive/.pst file
I hope this works for you. This is the same method I used to merge my old .pst file in Outlook.