As I am learning how FoxCart works, and I continue to use the manual encoding process, you might imagine the simple errors that were made like forgetting to include a field (category in this case )in the form. When I returned to the Manual Encoding tool, I realized that before encoding, I needed to include the "name" and "code" field without their HMAC hash codes, as well as the <form> tags in order to generate any newly encoded fields.
If the name and code fields are absolutely necessary to when generating any other encoded fields, my suggestion is to strip any existing hashes before generating the encoded form. This would enable users to copy their existing form fields right into the manual tool, add one or more new fields, and then generate the encoded version of those new fields without having to manually delete the hash values from the name & code fields (they're going to end up the same anyway).
If you can generate encoded fields without the <form> tag or the name field, all the better.