Sorry, nice try. The reason "they" that is, "the thems" call these "alternators" is because, internally, they actually produce AC, which in this case has no real polarity. In fact alternators in autos and trucks are THREE PHASE AC devices, and so that is why there are SIX diodes, instead of one or two.
The only way possible to invert the DC output polarity of an alternator is to take it apart and install the diodes differently. I'm not even sure if this is possible in many designs. In the old style Mopars which used six individually pressed in diodes, you would have to press all six out of the alternator and the insulated mount, and reinstall them in the opposite mountings. That is, swap the 3 negative for the 3 positive diodes.
Below is a simplified diagram of an alternator. Notice that 3 of the diodes have the "back of the arrows" (anodes) tied together and grounded. In the old style Mopar, these would be the 3 diodes that are pressed into the main part of the rear case.
Notice that the other three have the "bar" end (cathodes) tied together and feeding the output stud. These 3, in the old style Mopars, are the 3 that are pressed into the insulated sub assembly, connected to the output.
In the diagram below, a poor drawing of an early style Mopar, the 3 diodes in the little C or triangle shaped section near the top are insulated from the case, hooked to the output stud
The three around the big part of the case are grounded
