Do I have to use the mod api? ( Thermal Expansion )

Of course.  This way you have access to the blocks/items for the recipe.

I believe not necessarily, but it sure is the safest way: in case the block and/or item IDs change you won't have to change your code, too.

 

`ItemStack`, which is used in the `CraftingManager.add(Shapeless)Recipe()` methods, internally uses references to the blocks and items only to retrieve the corresponding item ID by itself. Ultimately, if you know the IDs of those blocks and items you want to use in your recipe, you could resign using the actual references and directly pass the ItemStacks to your recipe. But as I said, it does imply having to regularly check whether its still compatible with newer versions of the mod you're depending on...