Is it possible to add another chainring to Cannondale Trail SL 3?

There seems to be a place to mount a front derailleur on the seat tube of the bicycle. It also seems to have unused ports in the down tube for routing of another cable.

Picture source: https://www.cannondale.com/en/bikes/mountain/trail-bikes/trail/29-m-trail-sl-3

It is possible that the same frame design is used in different bicycle configurations which come with 2 front chainrings.

Given that your bicycle currently has a drivetrain with quite large rear 12-speed cassette range (from 11 to 51 teeth), there is a better solution.

Replace the front chainring with one that has bigger number of teeth. Currently the bike comes with 30 teeth at the front, so you should go with 32 or 34 teeth. There seems to be enough clearance between the ring and the chainstay to fit a slightly larger chainring.

Replacing just the chainring is definitely easier and cheaper than adding a front derailleur, front shifter, routing all the cables, another chainring and possibly swapping the crankset as the current one may be single-ring-specific.