Before Adding the CPU, check what kin dof license you have from you vCenter server.
you can navigate fromt he home>Licensing and chesck the appropriate license for ESX and see how many licenses are currently in use.
If you have additional licenses part from the ones that are currently in use, then you may proceed with your upgrade of H/W, if you do not have the licenses, check with the VMware support center, for any upgradepossibilities based on you financial constraints.