Is there a way to add a vm into an existing manual pool? if so, how?
Click on the manual pool name.
Then inventory tab.
If the VM is already exists in the same vCenter server, then edit the pool inventory and click on add and choose the new VM.
If you want to create a new VM(s) the ensure that the VM is customized properly, Join to domain (better use custom specification) and View Agent is installed.
You can also add multiple selected VMs.