Yes. You can assign any Sort Order value to any series or occurrence. However, we currently do not have a default button to assign a Sort Order value to all of the work orders from a specific customer.
I would suggest filtering your work order grid by the Customer that you would like to change and then manually changing the sort order for each series/occurrence related to that customer. Alternatively, you could create a custom button that loops through your grid and changes the Sort Order for each record that meets a specific criteria (ie. Name).
The best option would probably be to create a new field in the Customer table for Sort Order and link the field to the Activity table. This would require some customization in the Work Order List screen to use the field correctly. Let me know if you would like me to expand on that solution.
Hope this helps!
- Jason