It's okay. POST headers do not exist, but we do have HTTP headers and POST content. If you want to fake the data, you'd want to edit the POST content and not the HTTP headers.
Actually, a POST request is almost entirely in the HTTP header. You can, however, have the information being sent as the content, if you want. It's easier to do, but pointless and a bad idea for just a few text tuples.
Off the top of my head, a POST header (much condensed, I assume you're not interested in referer data, or acceptable charsets and such) is something like
POST thefanfictionforum.net/games.php HTTP/1.1
Host: thefanfictionforum.net
Content-Length: 0
Cookies: Allsortsacookieshereplusyoursessionid
uzqm: (I think)informationtosend.
It's not quite as easy to spoof, and you have more leeway for obscuring it than if you just send it straight as the content.
EDIT: Just checked. It actually just sends it as a plaintext POST content of content-length 0 (?!).
So it's REALLY easy to fake.