So would I do this [SELECT_MATERIAL:BONE] and then after it just list the tags that I need from steel?
And how would I go about doing it by changing the tissue?
EDIT: Also if I make the bones function as steel will this also change the skull to steel? or will the skull also use the new bone materials?
An easier way to improve a creature material using an existing material is to simply remove it and re-define it after the material body detail plan is invoked - so immediately following [BODY_DETAIL_PLAN:STANDARD_MATERIALS], you would do:
[REMOVE_MATERIAL:BONE]
[USE_MATERIAL_TEMPLATE:BONE:INORGANIC:STEEL]
This gives you actual steel bones, without having to manually specify all of steel's properties. If you still want it to act as bone for purposes of item-making, and insure against the material's density affecting speed, do this immediately afterward as well:
[SELECT_MATERIAL:BONE]
[BONE]
[SOLID_DENSITY:500]
Thickening an existing tissue layer should be a bit simpler - immediately after [BODY_DETAIL_PLAN:STANDARD_TISSUES] do something like:
[SELECT_TISSUE:BONE]
[RELATIVE_THICKNESS:3]
..or whatever thickness you'd like, where 2 is the default.
And yes, the skull is just a body part that uses the creature's bone tissue definition, and therefore bone material definition.
Edit: If you change a material that comprises a body part that's used for an attack, e.g. TOOTH, NAIL, etc it should determine the armor penetration capabilities of that part. Just keep in mind that of these, only TOOTH is defined in the STANDARD_MATERIALS body detail plan, and the rest are usually manually defined and assigned to the appropriate body part further down in the creature definition. Look for an existing [USE_MATERIAL_TEMPLATE:NAIL:...] tag, and point it to INORGANIC:STEEL (or LOCAL_CREATURE_MAT:BONE, if you've already modified the BONE mat) instead.
If you plan on modifying multiple creatures in a similar way, it's probably worth creating new BONE, NAIL, SKIN, SCALE, and CHITIN material templates, and maybe even a new materials BODY_DETAIL_PLAN, as that'll save a bunch of work in the long run.