How can I use the exact fonts from PowerPoint as the default in Impress?

[NickyRuggs]

impress font mapping

[salisburyk]

Only if you own a copy of PowerPoint and install it. As an alternative, you should know there are literally hundreds of quality open fonts in the public domain available now;

Google Web Fonts (can be easily used in documents)

Google Fonts

Check here for a review of other legitimate Open Source/Public Domain Font Sites
http://www.netmagazine.com/features/top-10-open-source-web-fonts

[qubit1]

In particular, take a look at the Liberation fonts (metric-compatible with many Microsoft fonts), as described here.