Hi Ratana,
You are right, my mistake. I'm sorry.
You can not add an item category that does not exist in original businees transaction (see OSS 1492963 and 1085921 Section 2.9)
You should create a new variant for unespecified posting business transaction (0000). This new variant should be similar to exiting one for 0300/0001. Then assign the unespecified business transaction with the new variant to the document types you want.
Also consider document splitting rules.
Best regards!