as far as I know such a table doesn't exists. Keep in mind that all outbound connections are translated (mapped if you like) to the primary IP address assigned to the ISA external interface. So, basically it is a N:1 mapping.
in the ISA MMC you can see the active sessions. That shows clearly which internal IP's and/or users have an active session. Also, if you want more detailed information, check out the ISA logs.
To get the most information out of the logfiles, I strongly recommend to enable the logging of all fields. In the MMC, go to the node Monitoring Configuration, then select Logs. In the details pane, right-click the applicable service and then click Properties. On the Fields tab, click Select All.