Contracting for Retrofits/Modifications

Not sure if this is the right forum for these questions, but here it goes...

Scenario: We are in the process of developing an IGE and J&A for the production of a widget, but would also like to include retrofits/modifications to said widget. Retrofits/modifications must be made by widget's OEM (from an integration perspective) as they own the Tech Data Package and it currently is not for sale. Is it possible to contract for an unknown quantity, unknown schedule, and unknown dollar amount of retrofits/modifications? In other words, can my requirement simply be for retrofit/modification services without specifically calling out each retrofit/modification? Is there specific language that must be used in the J&A for such a scenario?

If this is possible to do, has anyone done this recently? Would be looking for precedence to help bolster the possibility.

Thanks in advance...