Hi,
I understand that.
However, developping an import mechanism is not easy. It's hard work, while the perspective to gain users from it in the future is quite limited since RedShop is dead, as you said.
I'm afraid no ecommerce solution will be willing to develop such a migration mechanism in light of this.
The cost of such development is several thousand euros. It would be hard to get back the investment with a few users using it.
Of course, if someone wanted us to work on it for them, we could, but that's expensive.
That's why I proposed an easier path: Migrating just the products with an export/import
For the orders, you could leave temporarily an access to the orders history from RedShop. The user accounts are already compatible since they are in Joomla. So the main issue would be the users having to reenter their address during the checkout.