Gmail-label-like behavior for Outlook mail folder
You can do this with 'categories'. By default the categories in Outlook are just colors, but this can be changed. If you right click on a message, select the 'Categorize->All Categories...' menu item. The 'Color Categories' dialog will open which allows you to create new, rename, or delete categories.
If you then add 'CATEGORIES' as a column in the mailbox view you can right click and assign a category to the message. Multiple categories can be assigned to each message, which should give you the functionality that you want.
No way. Outlook operates with folders.