Can I use xml transformers to update InfoPath forms?

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