You can add new VM to a stored environment only. That means you should in MTM:
- Shut down your environment
- Select "Store in the library" from menu or context menu.
You can select location (the library to store into) on SCVMM machine then. - Go to the Library tab in MTM / Lab Center
- Open environment you have just stored
Now you will be able to add further VMs to it. After that you can just deploy changed environment.
But be aware: if you add/remove VMs to/from existing environment you will lose all existing snapshots of this environment.
So if you have some important snapshots you will have to consider how to deal with them before storing the environment.
PS: It's a pity it is not described nowhere neither in TechNet nor in MSDN... I found it out by chance...