Is it possible to "trace" a hall effect sensor on a PCB?
First, I think you got the principle of hall sensors wrong.

A hall sensor is a rectangular, thin piece of a conductor, through which a current is drawn from one side to the other:

             |
     --------+---------
     |                |
     |    current     |
-----+    ------&gt;     +------
     |                |
     |                |
     --------+---------
             |


A magnetic field perpendicular to the screen plane causes a force on the moving electrons, either upwards or downwards. This causes a voltage between the upper and lower terminal of the sensor, which should be measured with high impedance. The voltage depends on the material and (linearly!) on the current and the magnetic field.

So a hall sensor is a pure magnetic field sensor, but of course you can use it to measure the field of a second current, and so measure this current. There are many ICs available which do exactly this.

What you described in your first paragraph sounds more like a transformer, which can also be used to measure currents. But as they rely on induction, they only work for AC, while hall sensors can be used for DC, too.

This is also the reason why your idea about a second trace parallel to the trace to be measured does not work. You DO need a hall effect current sensing sensor. There are many different types of these sensors, but in most cases, the current has to flow through them. However, there are some ICs which are just placed over a trace, i.e. the current does not flow through the sensor, only through the trace beneath it.

For example this type from Melexis: