Have you already seen this article?
http://enterprise-solutions.swits.net/infopath2003/article.php?t=transform-data-
secondary-data-source-xslt&c=infopath2003

To implement the solution, you'll have to know XSLT and how to write code.
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S.Y.M. Wong-A-Ton

Hi Hans

First make a backup of your solution, just in case you screw something up in
your manifest file (been there, done that) ;-}

Within visual studio right click on the manifest.xsf file & select open
with, then select Source Code (Text) Editor.

When the file opens look for the dataObjects node & within there you'll find
a section for your secondary datasource, where you'll see field nodes.  All
you need to do is add another field node for your calculated field and away
you go, it'll now be selectable from within InfoPath.  Just save the
manifest.xsf file & open it normally.