You cannot receive Freesat on Sky boxes.
You can only receive Freesat on Freesat boxes.
The Freesat software is not available to run on Sky boxes.
However "free to air" channels, that require no viewing card, can be received on both Sky boxes and Freesat boxes.
Simply taking your sky card out of your sky box and you will get the same channels as you would on a Freesat box.
You have not and never needed a viewing card for free to air channels.
You have not needed a viewing card for BBC channels for over 8 years.
The only free Sky channels are Sky News, Pick TV and Challenge TV (the latter two recently move free to air. All other "free to air" channels are nothing to do with Sky at all.
There are some "free to view" channels that require a Sky card but no subscription: Five HD, Motors TV, Sony TV and a few others.
There is nothing wrong or illegal with the reception or watching of free to air tv channels from other countries in any country.
The use of a Sky viewing card outside the UK and ROI is against the T&Cs of the sky contract.