PPT is a proprietary format that is not well supported by other applications. Microsoft has made the specifications for the PPT binary format available, but I don't know how complete the specifications are, and many software developers may not have the time to implement an entire new set of specifications for a format that only PowerPoint uses.
On the other hand, the OpenDocument Presentation format (.odp and .fodp) is an open standard that is supported by OpenOffice/LibreOffice, NeoOffice, and Apache Open Office, as well as PowerPoint. If you save the presentation as ODP in PowerPoint, then Impress should have no problem displaying it as it looked in PowerPoint.
Then there's Office Open XML, which is Microsoft's pseudo-open standard. It's also supported by PowerPoint and LibreOffice. However, the specification is incomplete, and Office Open XML files still contain binary parts which only PowerPoint really knows what to do with. So that's another option, but probably not a very useful one.