My question is how does having free electrons make something a conductor?
I know that the flow or movement of electrons create a current but can't you just add free electrons (such as a battery) to an insulator and a magnetic field making the free electrons move, thus creating a current?
My question is how does having free electrons make something a conductor?
An applied electric field, such as that supplied by a battery, provides the force needed to move electrons which, in turn, means create current. Electrons that are not free are bound by electrostatic forces which oppose the force of the applied electric field thereby inhibiting current.
can't you just add free electrons (such as a battery) to an insulator
As pointed out in the comments, there are ways to add electrons but it is not particularly easy. Electrostatic charging one way but the current created by electrostatic discharge is usually brief. As far as batteries go, they don't supply electrons to a circuit. They convert chemical potential energy to electrical potential energy which is then used to push free electrons in the circuit producing current.
and a magnetic field making the free electrons move, thus creating a current?
Relative motion between magnetic fields and conductors can push and pull free electrons creating current in the conductors. Examples are moving a magnet in and out of stationary coil of wire, or moving a coil of wire over a fixed magnet.
Hope this helps.
As stated in Bob D's answer, batteries do not supply electrons to a conductor. The electrons are already present in the conductor, bound by weak electrostatic forces to the nuclei of the atoms of the conductor, so they are not entirely "free".
When a battery is connected to the ends of a conductor, it exerts a force on the electrons in the conductor (we usually say it generates a potential difference between the ends of the conductor which means the same thing as exerting a force on the electrons) and the electrons move readily since they were previously weakly bound.
An insulator also contains electrons but the electrons are bound to the nuclei of the insulator by very strong electrostatic forces. When a battery is applied to the ends of an insulator, the force exerted by the battery is not sufficient to separate the electrons from the nuclei, there is therefore no electron flow.
However, there are no absolute insulators as any material will conduct electricity if the force exerted on its constituent electrons is great enough to separate them from the nuclei to which they are bound. In order words, find a strong enough "battery" (which can supply a potential difference of tens of thousands of volts) and a substance usually regarded as an insulator will eventually conduct an electric current.
