Outlook - Keyboard shortcut for searching across all folders
Perhaps if you could clarify your request a bit.

Outlook for the Mac has several keyboard shortcuts for find here:

EDIT --> FIND --> (etc)

That all open a text input field for finding things. In fact COMMAND-OPTION-F brings up the search tab with the focus on the find field where you can type your query in.

Since this is so easy to find I can only conclude that you are looking for something more than that?

BTW, I just remembered that Outlook uses Spotlight to do it's searches, so you could do your search there and add criteria to the searches you need and save the searches as smart folders.