If this is an rrp mendel 3 then it can be upgraded to three extruders.
You will need a duet shield, extruder kit, bowden tube, thermister, heater cables....
Your exiting quickset hot end has apare slot so you dont need second , but you can choose to add a second hot end
All this is avilable in kits from rrp
RepRapPro Mendel 3 Tricolour
RepRapPro Fisher
-Carbon Arms
-Easy adjust Carriage+effector
-axis stiffness mods
HE3D -600 delta
-Duet 0.8.5
-PanelDue
-DC42 Height probe
-RobotDigg metal components
Simplyfy3D
RS Design Spark CAD