This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You can, but you'd have to pull them into a data mart via ETL. The latest version of Pentaho, 5.0, has the ability to do data blending against multiple data sources, but the performance isn't what you normally expect from an OLAP tool. However, it is very good for exploratory questions.