Rodrigo,
Yes, this is possible in FreeSWITCH. (I'm guessing it's possible in Asterisk, too, but I don't know enough about * to say definitively.)
The ideal situation is to use the "group_confirm" behavior that you mentioned and let party B press a key to accept the call. However, if you need the specific behavior of party B staying on the line and having party A automatically connect if B does not hangup then this will require a bit of programming. You could accomplish it with a mix of XML dialplan, phrase macros, and a dp script written in Lua, Perl, or Javascript. (Depending upon how simple the process is you might not even need a script.)
I'd recommend writing up the exact call flow in simple terms and then building the dialplan in small steps. Also, you could join #freeswitch on
irc.freenode.net and get some realtime assistance. I think people would be willing to help you if you agreed to put your example on our wiki. :)
-MC