Yes you can host machines that have been converted from Physical machines to virtual machines.
However you will need to get either the VMware Converter or something like Platespin to handle the actual conversation.
“Server running on another box”? Do you mean that this server is already virtualized and you just want it to be hosted on the ESXi host? Or Do you mean a physical server on a physical box?
a) You can either use vConverter / Acronis to make a duplicate of that server onto the ESXi host or use FASTSCP to copy those files that make up the system and then register that virtual server.
b) just use vConverter or Acronis to convert the physical box to virtual server to be hosted on the ESXi host.