I'm not very familiar with InfoPath forms, but if they are valid XML documents, then yes you should be able to update them with FME. You have 2 basic approaches you can take. You can either update the existing XML doc using the XMLUpdater transformer, or you can use load the InfoPath form as a template into XMLTemplater and then merge FME feature values into that. For more on XMLUpdater see: https://knowledge.safe.com/articles/22583/working-with-geodatabase-metadata-writing-to-xml.html

and https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Transformers/Transformers/xmlupdater.htm

For more on writing XML with XMLTemplater see the XML Tutorial article at: https://knowledge.safe.com/articles/30940/xml-writing-with-xmltemplater.html