If I understand what you are asking correctly, you want to use a SunTour Superbe Pro RD with indexed shifting. I don't think you will be able to get this to work, since the amount of movement for a given cable pull length is not going to match what is needed, regardless of whether you get a barrel adjuster installed or not.
By the way, I switched out the SunTour BL rear derailer on my Raleigh for a beat to death Shimano Exage 400 I got off a dead Trek 320 frame from Gilbert last week. The difference is pretty impressive. Though the SunTour is NOS and has those cute red replacement pulleys, it took a lot more effort to shift and required overshifting a bit and then backing off. After replacing the worn-out pulleys on the Shimano with some better ones from the junk bin (both lower ones, no Centeron), I found that this derailer shifts (in index mode, of course) crispy and quickly and requires MUCH less tension in the downtube shifter to hold it in place. This despite worn pivets and bent cage plates, etc...
You all reckon that's because Shimano combined the patent-expired slant paralleligram of SunTour with the top pivet spring of Simplex ? Shifts buttery !
Glenn Jordan - "hot-rodding" in Durham, NC