In addition to what has already been said, check the default zoning behaviour between the existing fabric and the new switch:
- defzone --show
Output will have one of two values, either noaccess or allaccess. To avoid a zoning conflict (and E-port isolation), both must have the same value. The best practice is noaccess.
Also good to check the admin guide for the FOS that you have and search on zone merge scenarios (last time I checked, Brocade publish about 8 pages).
Impossible to say if the merge will be successful, without further details (eg. is the current security policy using switch-to-switch authentication? Are virtual fabrics in use?)