NO, fixer contains other chemicals other than thiosulfate which could have undesirable effects on the developer. Even plain ammonium thiosulfate cannot be used. You have to use sodium thiosulfate or sodium/potassium thiocyanate.
sodium thiosulfate or sodium/potassium thiocyanate
Reversal B&W process is like any other photographic process - it's what you make of it. Haist had his formula for it 30+ years ago, good maybe for the films of the day - but not optimal. Some recipes produce very good result, some very poor.
The B&W reversal process for today's films is much different than yesterdays. Though there are basics, small details matter for the best outcome. sodium thiosulfate or sodium/potassium thiocyanate rob an emulsion of it's potential if it is used in a reversal B&W process. It is best to alter procedure or other baths to off-set the use of these agents. The image result is much more desirable.
I thought I'd cracked it at my second attempt, developing in Rodinal with no thiosulphate or thiocyanate. However, my third roll was a big disappointment, with a general muddy appearance that seemed to indicate that I needed something in that first developer. I've bought thiosulphate, but have yet to try it.
Even though you CAN NOT and MUST NOT use rapid fixer instead of hypo, you can still give it a shot . The point is, the amount of thiosulphate needed will be so low that the level of unwanted chemicals will be low also; you will be using just a few milliliters of concentrate per liter or so! I would think the ammonium ion is the biggest problem. But it's not going to explode so feel free to experiment...
Why I am telling this is because I have done this. When I was starting up in film photography, I experimented on reversal processing BW paper to create prints from color slides. As I didn't have plain hypo, I just poured a bit of fixer to the developer. It didn't explode, I didn't die and I liked the results; but no side-to-side comparisons done without the fixer!