Jav^H^H^H errr... I mean, Xtend [1] :). It does this through a mechanism called "extension methods" [2]. Kotlin has something similar called "extension functions", so I'm guessing it should be possible with it too.

1: https://github.com/eclipse/xtext/blob/master/examples/org.ec...

2: http://www.eclipse.org/xtend/documentation.html#extensionMet...

3: http://confluence.jetbrains.com/display/Kotlin/Extension+fun...