Since no one else answered, the solution is the same, just grab character columns instead and set the rows equal to the character you find fitting--"N/A" in the below case.
Just be careful, setting it =0 will change the column to numeric and will remove all the character data, however ="0" should be fine.
dt = Current Data Table();
nc = dt << get column names( Character );
For( i = 1, i <= N Items( nc ), i++,
nc[i][dt << get rows where( Is Missing( nc[i][] ) )] = "N/A"
);