How to add extruder to RAMBo Mini
I would say that the clue is in the name, MiniRAMBo. The Mini part is related to its diminutive size and reduced functionality.
Extending the board would require patching in a secondary board to the MiniRAMBo board via pins1 that may or may not exist, and would probably be more trouble than it is worth.
Looking at the board layout certainly shows that there are not any readily available connections for such an expansion.

If you want two extruders then the RAMBo would be a better bet, as it has 5 stepper motor driver outputs. It would certainly be easier, and quicker (as well as cheaper) than a MiniRAMBo plus a secondary daughter board).
That said, as the MiniRAMBo has two Z-axis controllers, it may be possible to repurpose one of them, if they are wired independently of each other and separately controllable. Although these lines suggest otherwise:

It has 4 stepper drivers and 4 mosfet switched outputs.

and

Differences from Rambo
MiniRambo has:

4 stepper drivers vs. 5


and

Motor Drivers
4 A4982 1/16th microstep motor drivers(2 connectors on Z for Prusa
Mendel and other dual Z printer designs)

So, it would appear that the two Z axis drivers are wired together.

1 By pins, I mean external board pins/connectors - rather than the pins of the µController itself