Press "A" and that will turn the window into Automation Mode. Then you can simply click with the pointer tool (tool 1) and create a slope from a point in the region to the end of the region. You can then use the "Automation Curve Tool" to create an automation curve rather than a straight fade out.
One thing about this approach is that if you move any region within that track, Logic will ask if you want to move the automation data, even if the automation data in question is a straight line.
Hope this helps!
JC