How can I add wheels to Hartman luggage?

Do you mean Hartmann luggage. Replacement parts are available from the manufacturer.

https://shop.hartmann.com/ReplacementParts.html

Can't just retrofit wheels. You can get a luggage cart that the luggage is strapped on.

If the luggage came without wheels, buy one of those 2 wheel carts that strap on your suitcase. You have to remove it to open your luggage each time but it's better than carrying heavy suitcases.

if the wheels have broken off I would look for replacement parts through the manufacture.

You can possible attach these to the bottom. https://amzn.to/2XGQHLu