What I actually wanted to ask is if I can adapt the writing style and not the story content from the forum. That I can say: "I know how to narrate my characters in this forum so I can do so in books I want to write."
I used to be a massive forum roleplayer some years ago, and now I mostly write novel and short stories. I've got a few good friends who used forum-based rpg too and they all tried writing at one point or another.
So, in short: you're in a very common situation.
Can you adapt the writing style?
Of course you can.
All the time you spent describing your characters, devising scenes, interacting with other people and introspecting will be useful while writing anything else. And old saying states that the first million words in an author's career are practice. In my point of view, your (ours) roleplaying adventure already gave you an headstart.
If you roleplayed a lot you have been exposed to a wide variety of writing styles, so that counts as reading experience too.
Thus said, there will still be some challenges. Roleplay gets you used to short, fragmented pieces, since most of the times you play your "move" and then wait for someone's else input (be it the master or other players). This was an issue for me: when I switched to write mostly novels, I tended to write short, uneven chapters. I often changed point of view in an erratic manner, i used a lot of time lapses, and I had trouble narrating the anything that wasn't pure action or interesting introspection.
So, you can certainly adapt your style and keep most of the skills you've acquired. In order to get better, identify what are your weaknesses.
Maybe, like me, roleplay has trained you to write in shortish, intense scenes, that you'll struggle to tie together. But it might be something else entirely. Maybe you'll find that weaving a plot alone isn't so easy as it was when having an entire community. Maybe you'll have trouble creating side and supporting characters.
Once you'll begin struggling, you'll know where you need to improve. In the meanwhile, get busy writing!