are you hoping to load the document into the document management part or attach it to a custom field? This is the XML for a custom field.
<Project name="my project" projectID="prj-001">
<CustomInformation>
<ColumnValue name="document-field-name">
<Documents>
<Parent documentLocation="/where/my/document/is">
<Document fileCreatedDate="2011-02-03T00:00:00"
hasAllParticipants="1" language="en" name="name-of-my-doc.doc"
numCheckoutDays="0" mimeType="x"
versioningEnabled="0">
<Version id="1" versionNumber="1" />
</Document>
</gel:forEach>
</Parent>
</Documents>
</ColumnValue>
</CustomInformation>
</project>
/Mark