Is it possible to add and control a booster pump on a DualSun pool heating system?

Yes, with a DualSun SLL solar controller.

Here is a diagram that explains how to wire and control the booster pump:  

Setting of the solar controller

To access the menu

•  Press and hold down the button
• Turn the rotary actuator to the desired menus
• Enter the menus by pressing the button

Parallel relay option

• OPARR = ON
• REL = R2
• RELR = R1
• INVERS = OFF

Configuration REL relay

• Relay R2 = ONOF

Example of contactor model: Relay 230V - 2 NO contacts - 9A (AC3) contacts for inductive loads (motors, up to 1300W).