[texhax] question about LaTeX use

[Karl Berry]

    Would You be so kind to ask the question: if it is possible 
    to paste a single A3-format page into LaTeX document?

If you are talking about inserting an A3 pdf, the pdfpages package might
be of some help.
http://www.ctan.org/tex-archive/macros/latex/contrib/pdfpages/

The geometry package might help with paper size stuff for your document
(that you're pasting into).
http://www.ctan.org/tex-archive/macros/latex/contrib/geometry/

Not sure this answers your question ...

best,
karl