adding and editing entries in inventory posting setup

May be the form is uneditable or you require some permission to insert/modify/delere entries (security roles & permission) otherwise I see no reason why you cann't do that.

One more thing of consideration is that the combination of Location Code and Invt. Posting Group Code should be unique. If a particular combination exist then you have the following choices:
a) Delete the line and create a new line for such combination
b) Modify the accounts in the existing line.

You have to further take into consideration the balance in the accounts of setup which have already been posted.