If you think about it, out of all the positions in the NRL, IMO #6 is the least specialised.
Brisbane have Lockyer converted into the role from fullback.
The Roosters will play Anasta there, who is more than capable of playing lock, and also Ashley Harrison could play there at a pinch who is also a lock
The Storm will play Scott Hill there, who's also spent time at lock
The Dragons will play Barrett there, who could play lock IMO quite well, and Timmins as a back up who has played lock/centre for most of his career but has repped successfully in the #6 jersey
Even Penrith, Campbell has established himself as a #6, but he couldn't get a regular gig in that jumper at Cronulla, going from fullback, wing, hooker...
Canberra will play Jason Smith there, who's played a fair bit of lock in his career
The Cowboys will probably put Jonathon Thurston to 6, he's a five eighth, but have dabbled with Myles in the past (centre, but now gone), Norton who's a lock, and Justin Smith who's a back rower
Some of the other guys regarded as more specialised, like Michael Witt just flat out aren't that good.
And as Cleary mentioned, a couple of the all time greats in Brad Fittler and Laurie Daley came into the #6 jersey after playing at lock and centre.
The true test of modifying a backrower into playing #6 is having kicking options surrounding him. I think the Warriors have that with Nathan Fien as number one kicker, Hohaia can kick out of dummy half, and I'd suspect Brent Webb will also move up towards the end of a set of six to play as an option kicker, he showed at various times on the Kiwis tour that his kicking has some potential, also either Gatis or Rovelli, whichever one makes the bench can kick as well. IMO Sione is probably the most all-round talented player at the Warriors, he can pass well, he can offload, he can step, and he seems to read a game relatively well. Hopefully though the off season he's worked on his kicking, although we have the previously mentioned options it'd be a feather in his cap to truly nail the spot down if he's developed a kicking game. I've seen him kick a couple of times for the Warriors, and do a good job at it. A couple of bombs last year from memory came up with very near misses on tries, notably one that was ruled a knock on against Louis Anderson in goal.
I reckon he can do it, and do it quite well. My only concern is where this leaves Jerome Ropati, who I think is also a very good all-round player but who just has to grow a kicking leg to grab the #6 jersey off Faumuina and establish himself long term at the club and as a squad member for the Kiwis.