I tried some po editor softwares. I wonder why none of do not allow creating a new po file from scratch or adding new entry to an existed one. Is there any hack so they let creating new entries?
I think the presumption is that you're not creating PO (or POT) files manually. Rather, you're using some sort of an automation tool that scans your source code and collects all strings to be collected into a POT file, to be translated, and you never edit your POT file manually. Such tools exist for many different types of files: I collect translatable strings from Delphi, Ruby-on-Rails, Windows RC files, HTML files, plain text files, etc.
Otherwise, you need to worry about keeping your translations in sync with the software. If POT file is generated automatically, it is always in sync.
This being said, nothing prevents you from opening a PO file in any text editor and editing it away, the way you like it. The format is plain text and is very simple.