I don't understand what you are trying to do.
You want to impute missing value with median or just bring median into original dataset ?
1) if you want to impute missing value with median, try this:
proc stdize data=have missing=median reponly;
by cluster;
run;
2)if you want to bring median value back into orginal dataset, try this:
proc summary data=have;
by cluster;
var var;
output out=median median=median_var;
run;
data want;
merge have median;
by cluster;
run;
3)Last, if you just simply impute missing WITHOUT median ,try PROC MI.
https://blogs.sas.com/content/iml/2020/12/02/score-external-logistic-model.html