There is now an API on the new Annots.SignatureWidget
class.
SignatureWidhet.CreateSignatureAppearance(Image)
https://www.pdftron.com/api/PDFTronSDK/dotnet/pdftron.PDF.Annots.SignatureWidget.html#pdftron_PDF_Annots_SignatureWidget_CreateSignatureAppearance_pdftron_PDF_Image_