Dave is absolutely correct: the caster is fully adjustable, as is the camber.

In fact, the SAME nuts and bolts that your [cough] "mechanic" used to adjust the camber are the ones used to adjust the caster. So to answer your questions: No, it's not true. To increase caster, the upper pivot point of the front suspension has to move aft; to make this happen, the eccentric nut - the one that adjusts camber (at the outboard end of the upper a-arm) - has to be moved further aft. To make that happen, someone with a wrench has to loosen the locknuts and then turn the eccentric nut until the desired caster is attained.

This technology dates back to the covered wagon era. For someone to assert that it's impossible is plain absurd. And they should have all their wrenches confiscated. This is an automotive technician, not to be confused with a mechanic.

You'll have a fair amount of trial and error getting started, but for the price of an alignment by a bunch of clowns who make up stories about why they can't do the thing you've hired them to do, you can own the thing that will allow you to do it yourself.

All you need is a couple wrenches, the green book (which nicely describes the process), and one of these (or something like it):

http://www.amazon.com/Longacre-Magnetic-Caster-camber-Gauge/dp/B003750D7U/ref=sr_1_1?ie=UTF8&qid=1428123450&sr=8-1&keywords=camber+caster

Best,

Cameron
Portland OR