Allow Multi field/column relationships to exist between different tables in Power BI Desktop

Basically, I am hoping for a way to allow a relationship to be created between two different tables that uses multiple columns to help get to a point of uniqueness (Similar to a multi field/column join in SQL). Currently, we can only select one option for merging and the best workaround is to merge columns in Power Query Editor until we get some messy (but workable) composite key. While this is an acceptable workaround, it isn't great in principle. Existing methods such as merging don't work in this instance given the quantity of data that would need to be brought in. Additionally, we cannot make the change in the source as that schema is not going to change for a relatively small number of problems.