Yes, you can use a shampoo bar as a bath bar. There seem to be subtle differences in their manufacturing according to to artisan web sites, but to me, it is all theoretical and not practical. I do it all the time and my skin has not fallen off or dried up or anything. I am sure someone somewhere will say that their skin became dry but I know of no health or cosmetic product in which someone somewhere did not have some reaction. Natural ingredients give me the confidence to use these products interchangeably. If you develop dry skin or dry scalp, which is unlikely, change products.
I haven't used "shampoo" on my head in at least 20 years. No problems that I know of. Mind you, my latest "soap" is called a "shampoo" maybe because of translation peculiarities, so maybe I've been using shampoo "on my body" for some time now with no ill effects...
I hear that using a bar soap might leave debris on your hair so some people use a diluted water vinegar mix to offest this but for me I use Ivory or Camay soap for my whole body and hair. I have a somewhat short hair and never bother with the vinegar mix routine. It sure is nice to have less things around the bath tub doing it this way and when travelling to carry less things as well.
I use only bath bar for years. I wash my hands with it, I have my shower with it and I wash my hair with it. And since I do this, I have no more problems with dandruffs. And it doesn´t care if the soap costs 0,49 or several Euro. The same result for me.