If you're asking this for reporting purposes, you can give users the ability to combine tables together for reporting using Explore Analytics. There's a video that shows some examples here:
Explore Analytics - Combining Data with Mash-up Views - YouTube
The advantage with that approach is that you don't have to make any code or table changes to ServiceNow, and reporting users can create as many different combinations of data as they'd like, because it's defined at the report level, not the table level.
