At the moment there are two ways you could approach this problem:
1. Export your transition group output to excel using the Reports tab of the Results Viewer and calculate the mean over a specified time period that way.
2. Use the Output Options Property to specify that you want interval mean output instead of annual output for transitions. Set your output interval to 700. You will get your mean values for year zero to 700 and for years 700 to 1000. To set this, you need to edit the Advanced | Output Options for the scenario before running it. Note that the graph will display a single point for each interval, representing the mean for the time period leading up to it.