Do you have CCX or CCE? What version?
Assuming CCX: Agents do not have an option to manually pull contacts from a queue. The selection criteria on the CSQ will dictate this. Along the same lines, there is no method for an agent to add or remove themselves from a CSQ. Supervisors can be granted this right within System Parameters though.
You will need to identify conditions with the Get Reporting Statistics step where the script should change it's logic (e.g. quantity of contacts waiting, longest waiting time, estimated wait time, etc). You can then queue the contact into additional or different CSQs based on these conditions.
As for session information: I'm not sure how that would assist you here; however, the Session steps are pretty straight forward. The Get Contact Info step will retrieve the contact's session ID for you. Note that you would need to trigger another application for the "other script" to start.
Cisco Unified Contact Center Express Scripting and Development Series: Volume 2, Editor Step Reference 7.0(1)
http://www.cisco.com/en/US/docs/voice_ip_comm/cust_contact/contact_center/crs/express_7_0/user/guide/uccx701edstepref.pdf
Edit: You could also have the script retrieve data from an external location instead of using the Get Reporting Statistics step. A supported database or XML file available through HTTP are two examples. This would allow you to provide an interface for controlling the behavior of the script to users. The behavior of the script and CSQs must still be predefined but you could allow users to selectively engage one or more predefined scenarios as needed.
