I had someone ask me about this today, and thought I'd share some information that I was able to come up with.
There are a couple of ways to go about it – unfortunately none of them are really a ‘quick’ plug and import. One is to use the Business Data Catalog within MOSS. It will allow you to integrate data from other applications (such as PeopleSoft).
The other would be to export the data from PeopleSoft as XML, and then import it into MOSS. Here’s a link with an example on how to do something like that. You’ll need to make some modifications if you are needing to make new entries: http://www.codeplex.com/MOSSProfileImport/ (there’s a link on there for instructions)
Personally, I’d recommend the BDC approach, just so that you aren’t always having to do the export/import process on a regular basis. You can learn more about the Business Data Catalog here: http://msdn2.microsoft.com/en-us/library/ms563661.aspx. Here's a good resource for integrating PeopleSoft and BDC: http://www.microsoft-oracle.com/Assets/pdf/PeopleSoftSharePoint2007Integration.pdf