You can register Orphaned VM which is not a part of vCenter Inventory or ESXi.
I am sharing two ways to do that.
First is by using vi client
1- Log in to vCenter Server
2- Select Desired Host in which you willing to add Virtual
3- Go to Configuration- Storage
4- Right click on appropriate datastore (Where orphaned virtual machine resides)
5- Navigate to virtual machine folder
6- right click .vmx file
7- Click Add to inventory .
After adding VM in inventory, when you try to power on VM, it asks whether the Virtual machine Moved or Copied. Select accordingly.
Using Command Line
Login to desire Host using SSH
Run Following Command:
vim-cmd solo/registervm /vmfs/volumes/<Datastore Name>/<Virtual Machine Name>/<vm name.vmx>
Above command will not work in ESX Host I checked both options in Vmware 5.5 and 6.0
For More Info check VMware KB