Hello,
This is not possible with current version.
However, we've build a new beta that allows this: ( https://dl.dropboxusercontent.com/u/13064071/plantuml.jar )
@startuml
skinparam classBorderThickness<<stereo>> 5
skinparam packageBorderThickness<<stereo>> 1.5
skinparam packageBorderThickness 0.5
class A <<stereo>>
class B
package p1 {
class C
}
package p2 <<stereo>> {
class D
}
Very few tests have been done and we are not 100% sure that syntax and new keyword are ok.
So feedback welcome!
Thanks for your suggestion.
About XMI, this is really frozen on our side. If you have something better than what we did and that can be easily integrated, we might be interested. Please contact us by mail (plantuml@gmail.com)