You'd have to manually edit the .vmx configuration file setting the option memsize = "12288" and when you start the Virtual Machine you'll get dialog box that says, "This virtual machine is configured with an unsupported main memory size of 12288 MB. VMware Fusion supports a maximum main memory size of 8192 MB. Continuing may result in poor performance." and it will have a check box that says, "Never show this dialog box again" that you can check.
Have a look at: Editing the .vmx file for your VMware Fusion virtual machine (1014782)