You can totally pass functions in a document stream

gistfile1.coffee

t2 = require 'through2'

one = t2.obj (doc, enc, cb) ->
  doc.speak()
  doc.foo = ->
    console.log 'foo'
  @push doc
  cb()

two = t2.obj (doc, enc, cb) ->
  doc.foo()
  @push doc
  cb()

one.pipe two

one.write {speak: -> console.log 'hai'}