You can only wear armour that is mentioned in the equipment list for each warrior type. This is because there is no 'Armour Master' skill like the 'Weapon Master' and 'Weapon Expert' skills. However, you can wear Gromril and Ithilmar (and probably darksteel too) if your warrior type can where Heavy Armour as these are all Heavy Armour just made with different metals. This is just like a warrior who can wield a sword can likewise wield a gromril or ithilmar sword are they are simply swords made from different metals. This is answered in the
Mordheimer Ultimate FAQ which cites its source as the "Games Workshop Official Mordheim FAQ". I have looked but I cannot find a copy of the original source.
For a Kislev warband, all warriors except for Streltsi warriors can wear Heavy Armour and therefore can wear Gromril and Ithilmar Armour.
Note that some gaming groups choose to house rule that Ithilmar armour is actually a type of Light Armour rather than a type of Heavy Armour as they feel that this makes more sense.