Hi WayneM and welcome!
To do this you need to make sure that you design your InfoPath Form Template from the XML file. InfoPath will infer a schema from the file which will allow you to open the XML file in the designer and edit it. There are some gotchas here since schema inference isn't an exact science. In particular, if your sample XML doesn't have repeating data, but other XMLs do, you'll want to make sure to use an XML file that has the complete structural possibilities. A better solution is to find the XSD file that was used by the previous process that created the XMLs and create your InfoPath Form Template from that. Then, you can be assured that the schema generated from InfoPath will be correct for all possible XMLs.
Unlike other Office Apps, InfoPath's XML structure is not proprietary. The only thing that InfoPath will add to your XML is a processing instruction at the top of the form. This is an industry-accepted practice to define "viewers" for the XML file, so it is legitimate. Everything else in the generated XML is non InfoPath specific XML.
Good luck!