In addition to what has already been said, check the default zoning behaviour between the existing fabric and the new switch:  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?)
Could some expert confirm to us that the following Brocade Switch Merging work without impact on the Existing Merged Production Pair Switches????  Thanks.  1. Existing Pair of Merged Switch (Firmware FOS 7.0.2) are of switchDomain 1 & 2 having the Merged Configuration defined and in effect. 2. There is a New Switch (Firmware FOS 7.0.2) of switchDomain 1 with no configuration defined and no configuration in effect 3. The New Switch switchDomain is going to be changed from existing 1 to 3 before connecting it to the Merged Pair of Switches 4. Please confirm that we can merge all 3 switches together as connected below without any lost  and impact on existing Merged Configuration.            switchDomain 1------------- switchDomain 2-------------switchDomain 3 (New Switch)
Brocade DS-6505-B Merging