The house main electrical panel is 200-amp.I want o add a 100-amp sub panel (for generator),can I put a 100-amp breaker to the main panel I order to feed the sub panel.on the sub panel I am not adding new breakers ,I just moving them there from the main panel.

You will need to use some kind of interlock kit or breaker.

See
Generator InterLock Kit - How does th Interlock Kit work? (http://www.interlockkit.com/ilkitworks1.htm)

As Harold noted, an interlock or transfer switch is critical to any generator install. You cannot use a generator without such backfeed protection.

Hire a professional to do this. Installing a generator is a simple task if you know what you are doing. You haven't conveyed that you fit that description.