Have a look at http://kb.vmware.com/kb/1000936
And http://kb.vmware.com/kb/1022152 is worth a read as well. All though this article applies to a Windows 7 guest, a lot of the concepts are the same.
http://kb.vmware.com/kb/1028042 talks about converting VMDK disk types using the vmkfstools command on ESX.
One solution you could try is as follows:
1. Copy the Workstation VMDK files over to your ESX datastore using WinSCP or something similar.
2. Once all of the VMDK files have been copied across, try converting them to an ESX type VMDK using the instructions contained within KB 1028042.
3. Once the new ESX-type disks have been created, in your vSphere Client select "New Virtual Machine"
4. Proceed through the wizard ensuring that you select the "Custom install" option.
5. When it gets to the section about creating a disk, choose the "use existing disk" option and browse to the VMDK file that you created.
6. If everything goes to plan, you will have a bootable virtual machine.
Hope that helps.