Try this:
* In the EXS editor, load in all the samples for one velocity group. Logic will map them chromatically for you. Once loaded, create a group for them.
* Adjust the velocity range in the group parameters.
* Load in all the samples for the second velocity layer. Again, Logic will map them chromatically for you, but starting after the last note of the first set.
* Make a group for them. Adjust their velocity range. Shift them, along with their root note, so they go "on top" of the first set of samples.
* Repeat for each new velocity layer.
In order for this automatic chromatic mapping to work though, each sound should have the same number of velocity layers. Otherwise the mapping will be off, and you'll need to shift the odd zone here and there in order to match them up.