PDFNet SDK offers a limited high-level support for XFA (i.e.Adobe LiveCycle) forms. Please keep in mind that XFA is a proprietary Adobe extension; it is not allowed as part of PDF/A and references to XFA may be removed as part of future ISO 32000-2 etc. As a possible solution your users could re-save PDF document with AcroForm/PDF compatibility in Adobe LifeCycle-Designer. After resaving the document using LifeCycle- Designer you will be able to render and process the file using PDFNet.
For more background and some thoughts on XFA Forms please see: