When you imported the workbook, did you check "Only take ownership if original owner cannot be found"? If you did, then I don't know where the workbook is. However, if you checked off the first option ("Always take ownership of imported workbooks"), then you will not find the workbook under the original user. If that is the case, import it again.
In order for an export | import to work properly you need to have access to the workbooks that are being exported. So they either need to be owned by the user doing the export or shared with that user. If you do not have access to the reports they won't be exported and therefore won't be imported. This is for security purposes.
The easiest way is to ask the original owner to share the workbooks with you then export them. When you reimport them you take ownership and can now share them with the original owner if need be.