Decrease headings automatically while inserting to main document

I have document with headings and generated table of content. I need to place hughe another document with headings like anex for main document.

main document:

Heading1 Title
 Heading2 SubTitle
Heading1 Anex 1


document to insert:

Heading1 TitleDocIns
 Heading2 SubTitleDocIns

I need headings of main document to be decreased while inserting into Anex 1 of main document and as result will get something like:

Heading1 Title
 Heading2 SubTitle
Heading1 Anex 1
 Heading2 TitleDocIns
  Heading3 SubTitleDocIns

How to achieve that?