How to generate translation file (.po, .xliff, .yml,...) from a Symfony2/Silex project?

After a long time searching the internet, I found a good one:

https://github.com/schmittjoh/JMSTranslationBundle

I see you've found a converter, but to answer your first question about generating your initial translation file -

If you have Gettext installed on your system you could generate a PO file from your "texts-to-be-translated inside the project". The command line program xgettext will scan the source files looking for whatever function you're using.

Example:
To scan PHP files for instances of the trans method call as shown here you could use the following command -

find . -name "*.php" | xargs xgettext --language=PHP --keyword=trans --output=messages.pot

To your question about editors:
You could use any PO editor, such as POEdit, to manage your translations, but as you say you eventually need to convert the PO file to either an XLIFF or YAML language pack for Symfony.

I see you've already found a converter tool. You may also like to try the one I wrote for Loco. It supports PO to YAML, and PO to XLIFF