Here's a much easier way.
You might have to change to Pages Palette Options to small icons in order to see what you are doing.
Open the main document (1).
Open the document you want to copy (2) and make the window smaller so you can see (1) behind it.
In document (2), open the pages palette & click on the icon of the first page you want to copy, hold down shift & click on the last page you want - they will all turn blue.
Let go of the shift key and click your mouse on one of the blue pages, hold down the mouse key & drag over on top of (1). You'll see a little fist icon as you drag.
It will look as if only one page or spread is copying, but when you click on (1), you will see the copied pages have all been added at the bottom.
Click on the 1st added page, shift, click on the last, let go of the shift, touch one of the blue pages & drag them all up to insert them between 104 & 105 - little arrows will appear before, after or in between the pages to let you know where you are. Let go of the mouse button & your copied pages will now all be exactly where you want them.
Unfortunately, the pages are always added to the bottom and your have to reposition them - you can't just add them right between 104 & 105 to begin with.