You want to take a currently configured system with Windows 2003 and install ESXi on the same hardware while moving the current W2K3 setup into a VM - right?
You won't be able to do this in one step but as Chris_K pointed out VMware Converter is the best tool to use for the conversion of the physical OS install to a Virtual Machine.
If I understand your specific requirements correctly then the way to do this is to build a temporary ESXi host with enough capacity to hold the Server temporarily. Then rebuild the main machine with ESXi and use converter again to do a Virtual to Virtual transfer of the VM back to the original server that is now running ESXi. You may even be able to use VMware Workstation or Player as the temporary target although I've never actually tried that, the Converter docs indicate that it is possible.
Note that you can resize volumes during a VMware Conversion, including the Windows System Volume and you can make use of thin provisioned disks so the temporary system can have less space available than the total volume sizes you want for the server once it's moved, you just have to able to provide a bit more space (a couple of GB should be safe enough) than the total amount of space that is actually currently in use.