Where are these hemoglobin values going to come from?
Normally you would not ADD a variable to an existing dataset.
The normal way would be create a dataset that has the data for the new variable and merge the two datasets to make a NEW dataset that has the NEW structure. Make sure both datasets have the proper key variables so the values of hemoglobin are adding into the right observations of the old dataset.
So for example if your existing data is called HAVE and each row is uniquely identified by the variable PATIENT and you have created a new dataset called HEMOGLOBIN that has the new variable with the hemoglobin values and also has the PATIENT variable then you can make an new dataset called WANT that has all of the data together.
data want;
merge have hemoglobin ;
by patient;
run;