var asmDef = AssemblyDefinition.ReadAssembly(typeof(EItemType).Module.FullyQualifiedName);
var module = asmDef.MainModule;
var itemType = module.GetType(nameof(EItemType));
var TestInjectedItem = new FieldDefinition("InjectedItem", FieldAttributes.Public | FieldAttributes.Static | FieldAttributes.Literal, itemType) { Constant = 114 };
itemType.Fields.Add(TestInjectedItem);
using var stream = new MemoryStream();
asmDef.Write(stream);
var newAsm = Assembly.Load(stream.ToArray());
var newType = newAsm.GetType(nameof(EItemType));

Console.WriteLine("Old type fields");
PrintFields(typeof(EItemType));
Console.WriteLine();
Console.WriteLine("New type fields");
PrintFields(newType);

//--------------------------------------------------------

void PrintFields(Type type)
{
	foreach (var field in type.GetFields())
	{
		Console.WriteLine(field.Name);
	}
}

public enum EItemType
{
    None = -1, 
    BasicCardPack = 0
}