Yes. You can use the FTPUserName property and the FTPPassword property.

Code:

function btnUpload_onclick()
{
   frmScan.DynamicWebTwain1.FTPUserName = "UserName";
   frmScan.DynamicWebTwain1.FTPPassword = "Password";

   frmScan.DynamicWebTwain1.FTPUploadAllAsPDF("192.168.1.20", "/Test/test.pdf");
}

References:

FTPUserName  FTPPassword  FTPUploadAllAsPDF