Posting to Billing document in KE21N

Hi,

Due to some complex business requirement, I need to post a value to a value field only for a specific billing document number using KE21N. However billing document number characteristics is not available in KE21N.

Why is it so? How can we post a value to a billing document profit segment manually then?

I cannot post this via SD module so please take note of that. The business requirement is such.