Are you using the beta client? In that case, a core mod won't work unless you add -localdataonly to the shortcut to the Ewom exe.
Items completely overwrite when you re-define them in a normal, non-core mod. Just put the following in a mod, and keep in mind you need a tech researched to see it in shops and in the unit designer.
<GameItemType InternalName="JourneymansCloak">
<!--Equipment Display Name/Description-->
<DisplayName>Journeyman's Cloak</DisplayName>
<Description>A journeyman's cloak protects its wearer from the weather.</Description>
<!--Equipment Type-->
<Type>Cloak</Type>
<CanBeEquipped>1</CanBeEquipped>
<Weight>8</Weight>
<CustomizationPointCost>5</CustomizationPointCost>
<IsAvailableForSovereignCustomization>false</IsAvailableForSovereignCustomization>
<!--Equipment Graphics-->
<IconFile>Journeyman'sCloak.png</IconFile>
<TintR>0</TintR>
<TintG>0</TintG>
<TintB>0</TintB>
<GameItemTypeModel>
<ModelFile>Gfx\HKB\Clothes\K_Male_Cloak_Mesh_01.hkb</ModelFile>
<AttachmentType>Skinned</AttachmentType>
<Color_Clothing1>43,23,23</Color_Clothing1>
<Texture_All>Gfx\HKB\Clothes\K_Male_Cloak_Patchwork_Texture_01.png</Texture_All>
</GameItemTypeModel>
<!--Equipment Production Requirements -->
<ProductionRequirement>
<Type>Resource</Type>
<Attribute>Gold</Attribute>
<Value>15.0</Value>
</ProductionRequirement>
<ProductionRequirement>
<Type>Resource</Type>
<Attribute>Materials</Attribute>
<Value>5.0</Value>
</ProductionRequirement>
<!--Equipment Prerequisites-->
<Prereq>
<Type>Tech</Type>
<Attribute>Equipment_Kingdom</Attribute>
<Value>0</Value>
</Prereq>
<Prereq>
<Type>Tech</Type>
<Attribute>Equipment_Amarian</Attribute>
<Value>0</Value>
</Prereq>
<!--Equipment Modifiers-->
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Moves</StrVal>
<Value>1.0</Value>
</GameModifier>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_Sight</StrVal>
<Value>1.0</Value>
</GameModifier>
</GameItemType>