Currently you can do it manually by editing the code like this:

        Effect01 {
            id: effect01
            opacityMaskSource: myMaskItem

        } 

However, there are no way to do this properly via property sheet. Once we have custom property sheets for components (QDS-11995) this issue can be addressed. 

Maybe some other effects would also benefit from this, so consider making this a generic feature of sampler type properties.

Note that it is necessary to set "layer.enabled: true" on the target item as well. Automating that might not be trivial.