Hi Will and welcome to the forum!
I am sure Excel would be the easiest to convert to XML. I just tried saving a worksheet as an XML Worksheet via File -> Save As -> XML Spreadsheet. After you do this, you should be able to add this XML document to your InfoPath form as a secondary data source and then populate from there. Even better would be to define a custom merge that would transform the "XML Spreadsheet" that Excel saves into the appropriate fiels for InfoPath. However, custom merges can get hairy.
I hope this helps, please let us know how you do it if you get it to work!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge