Solution1.Step1: Transfer the list content from Sharepoint to PPM: Create a table in PPM DB which will become a shadow of the sharepoint list. After than you could write a code in sharepoint that could connect to PPM DB to write down the list content to the table. This code could triger when you add a new item to the sharepoint list.
Solution1.Step2: Get data from the table to validation: In PPM, create a validation and write there a SQL which return validation content and which read data from the table.
Solution2.Step1: Transfer the list content from Sharepoint to PPM: You could create a view that call a table which contain the sharepoint list data from sharepoint.
Solution2.Step2: Get data from the table to validation: same as solution1.step2
I hope It is helpful for you.
IT Governance Professional & PPM Solution Architect