It is possible to design PGA using UAB. But we don't have any available component for UAB based PGA in Creator.
And please make sure that it is not possible to use 6 PGA in PSoC Analog coprocessor, if you want to route all the PGA output external to PSoC. Because, UAB does not have direct connection to the GPIO so it requires one CTB opamp to be used as the buffer. So if 2 UAB PGA's are designed, 2 CTB opamp is used as buffer so only 4 PGA's only be designed using PSoC analog coprocessor. This is true if you wish to route all PGA output external to PSoC.