Does it is possible to hack down into broadband service and get expencise plan for free?

The simple answer is NO. I suppose if you are a network engineer working for your ISP, then you might be able to do something, but for any other person then the moment you stopped paying for the service the ISP would simply disable it on your phone line or cable.

Attempting to access or use any network without the network owners explicit or implied permission is illegal. This applies even if you are within range of an unsecured WiFi network. Penalties if you are caught and prosecuted can include a fine, confiscation of all your computer equipment and/or a term in prison. Implied permission is where you go to somewhere like McDonalds that offers 'free WiFi'. They have to pay to provide the service, and they expect you to buy something during each visit that you use their WiFi. If you do not buy anything then you do not have 'implied permission'.

Why should other people pay just for you to steal from them?

Possible? Sure. Would you be able to do it or find someone who could do it? Not likely. If you found someone who could do it, would they ask for more money than the plan cost in the first place? Probable. Would it be discovered and you'd get in trouble for it? Almost guaranteed.

Hacking into someone's service is illegal in first place

Sure you can STEAL service and you can get a large fine or jail time along with the "free" service you steal!

Your best bet is to try and access an insecure network in your area and free load from them.

There is nothing free on this world.