The title says it all ;-)
I would like to be able to something like
\SI[certain-number]{18}{\winner}
or something like that
and get ``18'th winner'' in return
Edit: added use case scenarios
I Think I should explain why one would want to do this, so I've added this section to my question.
If one would like to explain either placements or events in time (a different sort of placement i guess) it is nice to have the ordered number together with the unit. here are some examples:
First from Mico's answer:
``They watched in exitement, as the teacher turned up the current and after passing the 22nd A mark on the dial the circuit began to smoke.''
Or:
``In a 22 km run after the 15th km line a lot of runners gave up''
I don't know for sure if these examples works in English, but they do in my native tongue, so i thought it weird that siunitx did not support it.
thanks for the answers.

\nth{18}\si{\winner}, but then I'm just not sure whether I should add a space in between or not ;-) – Thorbjørn E. K. Christensen Feb 09 '18 at 10:59siunitxquestion, this seems to me to be asking whether a unit can be defined which takes the value as an argument. This would seem to be incompatible with the\simacro and I don't really see whysiunitxneeds to be used to manage spacing, "3rd" isn't a value with units "rd". I suspect the correct answer is simply no, this is neither possible withsiunitx, nor something the package should attempt to provide. – Dai Bowen Feb 09 '18 at 11:12\SI; around the unitkmthere should be normal interword space. I don't know in your language, but in English it should be “at the 15 km line” (no ordinal). – egreg Feb 09 '18 at 13:44