is there a way to add custom variations?

[Courvoisier13]

Is there a way to define a custom variation list in Whoosh? For example, 'cat'->'caterpillar'

In the Whoosh python package, I am trying to add a custom variation equivalence. I can see that the variations are defined with a set of rules in whoosh.lang.morph_en but I can't see a way to add custom variations to it as a parameter to the QueryParser.
Is there a way I am missing?