What are the Ashkenazi and sifardi customs when kashering meat by roasting if it is permitted to use salt and/or other spices?
Shulchan Arukh, Yoreh De'ah 76:2 - Translation from sefaria.org:
If one wishes to salt the meat before roasting it (2)  and eat it without washing it, one may do so and we are not worried about the blood remaining on the salt. Some say this ruling refers to one who salted it, and then immediately roasted it, (3) but if it lay in the salt (4) the salt absorbs and is forbidden.  Therefore it should be well washed before the roasting. Rema:  Some say that roasting requires a preliminary washing (Ramban), and some say that it requires a slight salting beforehand as well (Rashi). The custom is to wash it first and then to lightly salt it when it is on the skewer, and to then immediately roast it so that the salt will not absorb blood.  Nevertheless, if it wasn’t washed or salted at all, or it was salted without having been washed first, and then roasted, it is still permitted. This is only if it did not sit idle in the salt (5) without being washed for the entire salting period, but if it did indeed lay idle for this amount of time before being roasted, it is forbidden.
As you can see, there are divergent opinions. I am thus inferring what the purpose of the salt is, which seems to be for koshering and not for flavoring. Thus, if you follow the opinion that it does not need to be salted before roasting, then your adding salt is for flavoring, and it seems that you can add pepper or anything else you like. However, if you follow the opinion that the meat must be salted before roasting, then, it seems the meat is unkosher otherwise, beforehand, and the salt is for koshering purposes. Thus, adding pepper before koshering would make the pepper unkosher.