Assign an additional parameter to the the C# class object

You could use an out parameter:

public Product GetProduct (out bool isExists)
{
    isExists=true;
    return new Product();
}

and call is like this:

bool isExists;
Product p = GetProduct (out isExists)

although it seems to me that isExists is the sort of property you might want to have in your Product class...

One way is to rework your method like so:

public bool GetProduct(ref Product product)
{
   ---
   ---
   bool isExists = true;
   product = new Product();
   return isExists
}

This way you can call the method like so:

Product product = null;
if(GetProduct(ref product) {
   //here you can reference the product variable
}

Why not use null?

public Product GetProduct()
{
   bool isExists = true
   if (isExists)
       return new Product();
   else 
       return null;
}

And using it:

var product = GetProduct();
if (product != null) { ... }  // If exists

Couple of suggestions:

Take a look a Dictionary.TryGetValue it behaves in a similar manner, if all you need is to return an object from a collection if it exists.

Product product;
if (!TryGetProduct(out product))
{
  ...
}

public bool TryGetProduct(out Product product)
{
  bool exists = false;
  product = null;
  ...
  if (exists)
  {
    exists = true;
    product = new Product();
  }   

  return exists;
}

If you have other properties you want to return along with the object, you could pass them in as parameters by reference

public Product GetProduct(ref Type1 param1, ref Type2 param2...)
{
  param1 = value1;
  param2 = value2;
  return new Product();
}

Another option is to group all objects into 1 pre-defined .Net class called Tuple

public Tuple<Product, Type1, Type2> GetProduct()
{
  return new Tuple<Proudct, Type1, Type2> (new Product(), new Type1(), new Type2());
}