It specifies that this output is for 16 ohm speakers. ... If I wanted to use this output as a "line level", would it sufice to use resistors in series to rise the impedance to around 47KOhm?
In modern audio systems (after the vacuum-tube-only era), impedances are not matched, but bridged. That is, outputs are made very low-impedance compared to inputs. (This approach has a number of advantages in simplicity, compatibility, and frequency response; its cost is electrical signal reflections that are insignificant for audio signals carried over short distances.) Therefore, you do not need to do anything for the impedance.
However, some speaker amplifier outputs may have their “−” side not connected to ground, but driven to negative voltages. If this is the case for your amplifier, then connecting it to a line-level input would short out the amplifier if the two devices have a common ground (and possibly produce odd signal leakage effects even if they don't).
The device you would need to solve this problem is an audio isolation transformer — a transformer that has a frequency response suitable for audio signals, and (as a starting assumption) a 1:1 turns ratio. I don't have any reliable recommendation for how to tell if you would need one, though a continuity test between the “−” output terminal and the line input ground would be a reasonable start.
Finally, you may find that the voltage range of the output is unsuitable, either too high or too low, so that the output is either too quiet or loud and distorted. In that case, you would need to either use a transformer with a different ratio, or (if it is too high and you do not otherwise need a transformer) a voltage divider, preferably placed at the input of the line-level device to minimize noise pickup (voltage dividers are always a tradeoff between being high-impedance and wasting power). A series resistor can also do the job but the value you need will depend on the input impedance of the line-level device.