Not sure if I get your question right, but I guess you want to include one full LateX-document into another. For this you could try the
includex-package. This package is not included in the standard TeX Live-distribution as it is in an unstable-branch of CTAN. In MikTeX it's included in package
frankenstein. With this package you can do something like this:
% this is file input.tex
Dies ist der Text von input.
Now you use this file included in the main document (this is not working with
\input-command!). It's important to load all packages used in the included files in the main file, as the preamble of the included files is ignored while processing main!
% this is file main.tex
Dieser Text ist in main.