Is there a way to make a specific part of a creature, for example the left hand, to be made of a different tissue material, like steel?
You can coat it in steel with something like [TISSUE_LAYER:BY_TOKEN:LH:HANDSTEEL] (where HANDSTEEL is a local creature tissue you've made out of INORGANIC:STEEL) but making a standard bodypart entirely out of an odd material would require you to make your own body plan. Basically you build a BODY of all-but-the-left-hand, apply standard tissues to it, add a separate [BODY:LEFT_HAND_OF_DOOM] which is its own tiny body plan, and apply the above tissue layer. Since there are no other layers, the entire bodypart will be made of the stuff.
Note that this is just the hand. It gets a bit more tedious if you want the fingers as well, but the same principles apply.
Actually, SET_LAYER_TISSUE lets you do this without mucking with the bodypart raws. It's a funny obscure tag that I don't think is used anywhere in the stock raws but it works fine.
Tested it with this example:
[SELECT_TISSUE_LAYER:ALL:BY_TOKEN:LH]
[SET_LAYER_TISSUE:SPLEEN]
And I successfully tore the spleen tissue and bruised the spleen tissue.
I did do that with one hit though so it seems to game might handle the combat text weirdly when you have multiple layers of same tissue. Depending on your needs you might still have to go with Dirts's method.