The only way I can think of doing it is to break the document into 3 sections. Bit difficult to explain, if I find a link to a tutorial I'll post back, but start by putting the cursor at the end of page 6 then under 'Page Set-up', look for 'breaks' and select 'section break', do the same at the end of the next page then on both of those pages unclick 'link to previous' in the toolbar. Then you can auto number pages 1-6 and 7-16 and manually insert 6A in the header.
"I've always been mad, I know I've been mad, like the most of us..." Pink Floyd
The information on Computing.Net is the opinions of its users. Such
opinions may not be accurate and they are to be used at your own risk.
Computing.Net cannot verify the validity of the statements made on this
site. Computing.Net and Purch hereby disclaim all responsibility
and liability for the content of Computing.Net and its accuracy.