Customer has a cutler hammer panel 200 amp service,the main disconnect is on outside of building he wants one in panelin his garage, how do I do this without replacing whole panel

Not much information to go by.

What size panel is needed in the garage? 100 Amp?

Can the existing 200 amp panel accpet a 2 pole 100 amp circuit breaker?

What is the new panel for? Will it add any substantial load?

Will the existing 200 amp service handle the new load?

Have you done load calculations yet?

Are you an electrician?