Hi,
What you want requires changes that can't be made through SysConfig or any other configuration file.
There are 2 ways you could do exactly what you asked:
1. Change the code in both a) the Perl template file primarily responsible to generate that grid (CustomerTicketOverview.dtl) to add the column you require and b) the Perl module computing data for your template file (CustomerTicketOverview.pm) so that the new column is provided with required data (out of the box, CustomerTicketOverview.pm doesn't provide it).
2. Create an event module hooked up at the TicketAccountTime event. In this module's code you compute the data you require when time is accounted for a ticket and then you store it in a ticket dynamic field. Then you can configure OTRS through SysConfig (Ticket::Frontend::CustomerTicketOverview###DynamicField) to show this dynamic field in the customer grid.
There is another way to achieve an effect close to your request that doesn't require coding but involves other problems:
3. Create an agent account for your customer but BEWARE of how you configure security so that this agent only gets access to STATISTICS. Then create a report that contains the desired data and make this report visible to this agent.
Needless to say, (1) and (2) are non trivial and require some decent development knowledge and at least a little bit of Perl knowledge. (3) doesn't require coding but it requires that you trust your customer to be decent (not probe for flaws in OTRS once he has limited access to agent UI) and it also requires a strong discipline on your part when you configure OTRS so that the customer never gets unintentional access to other agent/admin OTRS areas.
//bogdan