Z visa for an official passport holders

I am writing from Bangladesh. I am a Government Officer of Bangladesh and I am holding an "Official Passport". I want to go in China through Z type for 1 year. I have Work Permit issued from " The state Administration of Foreign Affairs P. R. China" and a invitation letter issued from Yunnan Academy of Agricultural Science (YAAS). I went to Chinese Embassy in my country. They didn't give me the Z type. They told me if you want to go PRC with Official Passport you need other documents. They also told me, you can go with this documents but you have to change your passport " Official passport to Non- Official passport".

Now, my question, I want to go PRC with Official passport. So, who will issue my invitation letter. I will be pleased if you give me suggestion

with regards
Dr. Md. Ashraful Alam
Scientific Officer
Wheat Research Centre
BARI, Dinajpur