[plt-scheme] [redex] extending languages

Hello,

I am wondering what would be a recommended way to play with 
extending languages in plt redex. 

[In short, I am facing the so-called expression problem.]
When a language is extended with new constructs, then I need to extend
both the reduction-relation and associated metafunctions, such as one
for substitution. As a reduction-relation is extended, 
rules inherited from the original reduction-relation should use 
extended metafunctions. Similarly when a metafunction g, which may call
another metafunction f, is extended,
clauses inherited from original should call the extended version of f. 

Is this doable within redex facilities?
Or, should/could I use PLT's unit system?

Best,
Keiko