Do you mean making other primary tags? You can do so in the Admin panel, under Tags. Click on Create Tag button and fill in the details.
Or are you talking about posting with multiple primary tags? This isn't possible, only one primary tag is allowed, but you can add as many unattached secondary tags (those that aren't nested under a primary tag) as you wish. You could even make ALL your tags (including the first) secondary tags if you like.
