Word can easily handle a 1000-page document.
What you are describing would be ideal for use of the Master Documents "Feature" if only it worked. (Read the page linked, and maybe links in that page before you even think about using this on anything valuable.) IncludeText fields should do what you want. This works best if the files are based on the same template and use the same Styles for the same purpose.

Again, I'm not sure why this can't be in a single file.

Charles I'm sure word can handle 1000 pages. That is also not my question.
What I like is to have for every chapter a separate file and combine that to one report. If I make a report I can pick the chapters I need en write some new chapters. I can use the copy/past function. But that is really stupid to do this for 100+ page documents.
Why I like to have several files it that I like to have a file with only the introduction one file with the way I did the experiment etc this are mostly the same. Than I write the results for that test with conclusion and than I end with a standard text. I can make every document by using a standard report but if I chance something (in the introduction) I have to do this in all reports but if I have a file only with the introduction I just reposes the excision reports and automatic the update introduction textfile is used.

I you have worked with LaTeX (not latex) you know what I'm talking about.
Small section from a TEX file in LaTeX
[code]
\include{010-inleidingV1_0}
\include{020-storagesV1_0}
\include{030-materialsV1_2}
\include{040-conceptsV1_0}
\include{050-results20140204}
\include{999-conclusion20140204}
[\code]

Between { } are separate text files