The title says it all; a friend asked me this (he wants to put it on some sports gear or something) and since I´m getting a bit confused with different search results I´m asking here.
Do you want it in specifically Kanji or are you willing to accept katakana?
Katakana may be the way to go. That alphabet (technically it’s a syllabary, not an alphabet) is used to write foreign words in Japanese and sometimes gives those words a bit of cachet. Given the target audience of extreme sports, I’d go with katakana.
OK, katakana then. 
Well, first let’s take a whack at it in roman characters. I’d expect it to be something like:
e-ku-s’ta-ri-mu su-po-to
Try this page for the katakana equivalents. It doesn’t show a stand alone “s” character, but I thought there was one (like the stand alone “n”). Might have to use “sa” for the “s”.
Oops. “su” for “s”, not “sa”.
I’m sure a bona fide *nijon-jin * will come by shortly to improve on my first cut.
Ale, i have sent you an email with both the katakana and one possible Kanji form for Extreme in it, i hope it helps.
I’d just like to add that I am a bona-fide Igirisu-jin not a * Nihon-jin*.
Oh and (sorry to post again) the Roman equivalent of the japanese i have emailed would be in the first image “ekkusutori-mu supo-tsu” and in the second “hijou supo-tsu”
Thanks a bunch, I´ll check that out.