If it's text, distance field fonts provide a very fast way to do it. See here
However, there is a way to add an outline at runtime to any kind of object: by "abusing" the GlowFilter slightly. Kiwicode mentioned this as the "alpha > 1" hack. 🙂
The GlowFilter is implemented in a way that when alpha exceeds 1.0, it acts as a strength-multiplier, making the edges of the glow harder.
For example, to add a black outline to an object, set up the filter like this:
object.filter = new GlowFilter(0x0, 50, 1.0, 1.0);
Note that "alpha" was set to "50" here, even though it's normally in the range 0-1. 😉
It's definitely worth a try!