To be clear, you are using Power Pivot to import data from SSAS into Excel, then uploading that workbook to Power BI? As far as I know, this should work. I guess you've seen these pages, and the links to the troubleshooting tips?
https://powerbi.microsoft.com/en-us/documentation/powerbi-refresh-excel-file-onedrive/
https://powerbi.microsoft.com/en-us/documentation/powerbi-service-excel-workbook-files/
Chris