How create dialects with XText

Question

The project I'm working on has a custom file format, with a pre-defined structure. The structure is really simple and generic (and I cannot change it): it is composed by (nested) commands and typed properties.

Using this structure, several dialects have been created. The dialects are an "instantiation" of the generic grammar, and specify the name and the meaning of commands and the expected properties.

I created a model with EMF for one of these dialects, and I would like to reuse XText to easily create a professional text editor and be able to read and write my model into the correct format.

Now I have a choice. On one side, I can directly target the dialect, and mix in the same grammar the concepts from the custom file structure and those from the dialect. On the other side, I can create a grammar describing the file structure, and on top of this I can describe my dialect.

Which way I should follow? I think that the latter is the best one, but how can I create a grammar describing those two layers?