If you do not need to don't deploy another cluster, as long as you meet all requirements described in SRND you can have up to 20 CUCM servers in a single regular cluster, 8 of which can be call processing nodes. To add the server you will need to know the existing cluster security password and install exact same version, while doing it all phones will be reset, so make sure you plan for it. Once the server is installed and services are started you will need to plan on which CUCM group you want to add this node to or if desired build new CUCM Group and add to appropriate Device Pools for this server to be used. You'll also need to update any integrations, i.e. gateways, voicemail, etc. that should point to this server.