You'll have to migrate the disk to a disk that's already set up in split configuration.
Create a disk with the same or more disk space allocated to it, when creating it make sure that you choose the option to split the disk.
Ensure that the virtual disk being changed does not have any snapshots associated with it.
- Power off the virtual machine.
- Open a command prompt with Administrative privileges.
- Navigate to the product's installation directory.
- Run this command:
vmware-vdiskmanager.exe -r <filepath of original disk> -t 1 <filepath of new disk>
To determine the location of your existing disk go to VM > Settings, click the Hardware tab, and select your hard disk. The disk file is listed at the top, on the right side. If no path is provided, then the virtual disk is in the same directory as the configuration file