CustomConvention.cs

using System;
using System.Linq;
using Fixie;
using Fixie.Conventions;

public class CustomConvention : Convention
{
    readonly Ploeh.AutoFixture.Fixture autoFixture = new Ploeh.AutoFixture.Fixture();

    public CustomConvention()
    {
        Classes
            .NameEndsWith("Tests");

        Methods
            .Where(method => method.IsVoid());

        Parameters(type => autoFixture.Create(type));
    }

    private void Parameters(Func<Type, object> parameterByType)
    {
        Parameters(method =>
        {
            var parameterTypes = method.GetParameters().Select(x => x.ParameterType);

            var parameterValues = parameterTypes.Select(parameterByType).ToArray();

            return new[] { parameterValues };
        });
    }
}