SAS allows one type of data per variable: numeric or character. So you can't "mismatch" the data.
It really isn't clear though I am going to assume that C is a numeric variable. You can get SAS to display text like 'NA' using a format associated with a specific value. From context I would use a "special missing" which are values like .A .B … .Z or ._
The use a format to display the text NA as desired for that value.
proc format libray=work;
.N = 'NA'
format x MyNA.;
proc print data=junk;
This avoids attempting to assign text to what looks like it really should be numeric.
Or you could just use the generic missing value of . instead of the ".N" without any special format. The missing value would not be used in most calculations (it is less than any value though) and you would likely know that missing values come from this sort of assignment.