I am encountering the same issue.
There are expense claims created by employees who pay for things with their personal credit card, and those expenses are signifiant enough to warrant a purchase invoice (e.g., our operations person pays on their personal card all of our sales team’s cellular bills).
I like the workaround solution you provided SecA.
It would be wonderful to be able to relate specific expense claim items to a purchase invoice. Even if it’s not on the roadmap, I still have nothing but thanks and praise for the ERPNext team for this wonderful product.