If you can open the OST file in off-line mode, you can then create a PST and simply copy the contents of the OST over to the PST. I would like to suggest some manual tricks to convert OST to PST format. Read this blog to understand how you will use “Export option” of Microsoft Outlook. You can also try archiving.
The trick here is opening the OST at all, since it is typically tied to a specific profile or Exchange Server account settings. If that account is not available, then you may not be able to open the OST.