There are a lot of pages on the internet about how to use hipchat with bitlbee.
Because of this I'm not going to explain that. Do your google foo and find out.
There is one really annoying thing: people tend to fill up your buffers with join/part messages.
Now there are some pages out there that tell you how to fix it with irssi but did not find a weechat tip.
So see here, this is how I did it.
In my case, I added the bitlbee server with
/server add bitlbee my.bitlbee.server'
Now, to get rid of the join/part messages type:
/filter add joinquitbitlbee irc.bitlbee.* irc_join,irc_part,irc_quit *
And no more annoying join/part/quit messages that are very common with all the IM services out there.