As far as I know, OST files are protected, so the only way to turn them into PST files would be to first log in using the original MAPI, then open the OST file in question and finally export it to the PST format. I know that this method isn't that convenient, but it's guaranteed to work.
Alternatively, there's a tool called OST PST conversion tool that boasts about offering exactly the functionality that you're looking for, but I haven't tried it for myself, so I can't guarantee that it will work.
