in default configuration in freeswitch, when a call falls into the queue and is routed to an agent, he immediately answers, I want to change that ...
so is there a way (is this possible) for off-hook agents (always connected agents) to acknowledge the call before bridge (for example by pressing DTMF code) ?
Yes, this is possible. In Freeswitch callcenter module (mod_callcenter.c), you can change it according to your need before bridge to agent. And you can write the code for sending dtmf to off-hook agents. I used to do the changes as per my requirement and I also wrote many custom specific modules.