IDKit SDK and ExpressID AFIS forced image format to PNG when saving in IDKit/AFIS database until version 2.24. From this version user programmer can choose the format of images stored in IDKit SDK and ExpressID AFIS database using CFG_DB_IMAGE_FORMAT configuration parameter (or IDKit.DBImageFormat property in .NET).
In order to convert fingerprint images into WSQ, use IDKit SDK to stream the image in WSQ format and save in your application database. When the image is in application database, you have full control over it. You then use your normal database interface to save/load the WSQ image. To obtain image in WSQ format using IDKit SDK, use the following C# code:
// ... obtain fpImage
Fingerprint fp = new Fingerprint(fpImage);
MemoryStream memStream = new MemoryStream();
byte wsqData = memStream.GetBuffer();
// ... save wsqData in application database