I am in the process now of setting up a second model for Customer Expansion that would include Add on and Upsell. We have it planned out but not executed. Below is the basic premise which needs testing and more thought:
Three Step Model
1) Someone enters the model when their Oppty becomes WON.
2a) When they are added to a new Oppty w an Upsell Oppty Type, they enter the Upsell model phase.
2b) When they are added to a new Oppty w an Addon Oppty Type, they enter the Addon model phase
3) When that Oppty becomes WON, that's the final step.
Interested to hear from others on how they are addressing this next gen of model measurement.