Can I convert a 1x9 speed Madsen kg271 cargo bike to a 2 or 3 x 9?
This link says the frame has the necessary cable stops for a front shifter and front derailleur. 

You'll need a new crankset (you can find 9 speed+ triples or doubles pretty cheap these days). This may require a new bottom bracket as well, depending on which crankset you choose (I'd guess whats on there is just standard square taper) and the chainline required. 

You'll also need a front derailleur. Front derailleurs come with a slew of selection criteria: top/bottom/dual/side swing pull (the cable comes from the top/bottom/top or bottom/ side), where the derailleur is mounted (top swing (the derailleur swings above the clamp), bottom swing, direct mount/e-type variations, braze on, band clamp) and if its designed for a double or triple crankset. Then, you need the derailleur to have the appropriate chainring size constraints. There's a good overview here of types of front derailleurs. The mount type necessary is dictated by your frame.

You'll also need compatible shifters (I'd suggest buying a front shifter from the same manufacturer and mountain/road type; They get a bit weird with these 11 speed-intended FD's though). 

As mattnz pointed out in the comments (thanks!), you may also need a new rear derailleur (which has to be compatible with your shifter), in order to have the capacity for the chain in the new chainrings. 

I'd suggest heading to your bike shop, and having them determine what kind of FD goes with your frame at the least. You'll need a few specialty tools for this -- bicycle cable cutters, a crank puller and possibly a bottom bracket tool.