Forth is very free-form, it doesn't require that your words be 'ordinary' in that sense. The : word isn't the only one that can consume from the input stream, neither is it the only word that can define words (such words are called defining words). The CONSTANT word, for instance, is a convenient syntax for defining a word to push a constant value onto the stack. [0]
You can even define your own 'mirror' of the : word, which involves some fiddly management of Forth's state, but it's doable. [1]
See also [2] on Forth's execution model and [3] on defining your own defining words.