Inserting manual page breaks is certainly possible but doing so has adverse effects:
1. It splits paragraphs in two, which will cause an auto-numbered paragraph to become two sequentially-auto-numbered paragraphs.
2. It splits tables in two, which can be disastrous for tables using header rows to populate multiple pages - it also messes with cells that span page breaks.
3. It prevents Word from re-flowing the document content properly when opened on a computer using a different print driver to the one on which the page breaks were created. This can result on a new page being created with just one or two lines of text (containing as few as a single word) and/or other pages not being filled (having just a few words (if any) on the last available line for a paragraph that continues on the next page.
I'm sure other contributors here can list more adverse effects, too - the above spring readily to mind.

I'm not sure what you mean by "some soft page breaks missing" - soft page breaks, as such, don't exist in a Word document - they're nothing more than a display construct that shifts around while the document is open in response to edits.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]