How can i manually relate pages?

2 possible solutions spring to mind, both fairly straightforward:

  1. Use a custom taxonomy to create groups then add the accessories and main product to the same group
  2. Add an interface that updates the post_parent field to mark items as accessories for a specific product

With either of these approaches you get a fairly easy way to group the products together although the taxonomy approach allows for more flexibility if there are accessories that could be used for multiple products.