You are saying the you want to use the associated item as xpath constraint. How/where do you want to use it exactly? Please edit/enhance your question or add a comment.
If you run a microflow on your main object holds the associated object, so you could use on change logic to get the required effect?
On a (input) reference selector, you can add constraints using existing associations. eg. When adding a person, you first select a country, in a second dropdown, you can only select the provinces/states related to that country.
Once you provide a bit more info, I might be able to guide you in the right direction.