AssertExtensibility.cs

using System;

namespace SomeUnit {
    public class AssertHelper {
        public static AssertHelper Assert { get; } = new AssertHelper();
    }
}

namespace SomeUnit.Extensions {
    public static class AssertExtensions {
        public static void SomethingNew(this AssertHelper assert) {}
    }
}

namespace Tests {        
    using static SomeUnit.AssertHelper;  
    using SomeUnit.Extensions;
    
    public class Tests {
        public void Test() {
            Assert.SomethingNew();
        }
    }
}