How do I extend Markbass combo?

Here's the owner's manual: 001054_e_markbass_little_marcus_series.pdf

POWER AMP (MPT): 500W @ 4Ω / 300W @ 8Ω
The combos speaker load is 8 ohms, so you can add one additional 8 ohms cabinet: minimum ohm load for all Markbass heads is 4 ohms.

I agree that Markbass didn't make it easy to add an extension cabinet to this combo. If you don't mind voiding the warranty, I'd install a Speakon female connector on the combo instead of the 1/4" jack. This would allow you to run Speakon from the amp to the extension cabinet, then from the cab to the internal combo speakers.

You could also do it with an adaptor like this one, but the more connections you use the greater the points of failure.
Hosa GSK-116 1/4-inch Female TS to Neutrik speakON Speaker Adaptor Cable | Sweetwater

 

It looks like it's a bit more complicated than it should be. The combo speaker is hard wired to a plug with no female connection on the cab.

To do it that way, one would need a cable extender or would need to add a connector to the cab.

 

As your photo clearly illustrates, there is only one speaker output on the back of the amp.
There are several versions of the CMD 102, each with different speaker outputs.

• CMD 102P - One Speakon combo, one 1/4" outputs
• CMD 102P IV - One Speakon combo, one 1/4" outputs
• Marcus Miller CMD 102 250 - One Speakon combo output
• Marcus Miller CMD 102 500 - One Speakon combo output