I'm not an expert in min-liberty's but I do have the same flasher.
The flasher has 4 ports and as you state, all ports are taken. You'll probably need to "tap" in to one of the channels with the new module. I assume you'll be putting this new module in the center section (since it is the only spot). By tapping in, the center section is going to flash at the same time as one of the ends.
You could re-wire things and make the 2 ends alternate with the new module in the center.
Hope this helps.
EDIT: As another member stated, if it has its own flasher, you could just tap in to the power going in to the flasher. The light head will not be synced with the originals but, another option.
-Rob