Using a List as an intermediary is the easiest way, as others have described, but since your input is an array and you don't just want to keep the data in a List, I presume you might be concerned about performance.

The most efficient method is likely allocating a new array and then using Array.Copy or Array.CopyTo. This is not hard if you just want to add an item to the end of the list:

public static T[] Add<T>(this T[] target, T item)
{
    if (target == null)
    {
        //TODO: Return null or throw ArgumentNullException;
    }
    T[] result = new T[target.Length + 1];
    target.CopyTo(result, 0);
    result[target.length] = item;
    return item;
}

I can also post code for an Insert extension method that takes a destination index as input, if desired. It's a little more complicated and uses the static method Array.Copy 1-2 times.

Using a List as an intermediary is the easiest way, as others have described, but since your input is an array and you don't just want to keep the data in a List, I presume you might be concerned about performance.

The most efficient method is likely allocating a new array and then using Array.Copy or Array.CopyTo. This is not hard if you just want to add an item to the end of the list:

public static T[] Add<T>(this T[] target, T item)
{
    if (target == null)
    {
        //TODO: Return null or throw ArgumentNullException;
    }
    T[] result = new T[target.Length + 1];
    target.CopyTo(result, 0);
    result[target.Length] = item;
    return result;
}

I can also post code for an Insert extension method that takes a destination index as input, if desired. It's a little more complicated and uses the static method Array.Copy 1-2 times.