Appending Data with Proc Append
Proc Append is more efficient. Variable names and types must match on both data sets!

proc append base=classlib.htwtdata data=moredata;
run; 

SAS Log:

NOTE: Appending WORK.MOREDATA to CLASSLIB.HTWTDATA.
WARNING: Variable h was not found on BASE file.
WARNING: Variable w was not found on BASE file.
WARNING: Variable g was not found on BASE file.
WARNING: Variable height was not found on DATA file.
WARNING: Variable weight was not found on DATA file.
WARNING: Variable gender was not found on DATA file.
ERROR: No appending done because of anomalies listed above. Use FORCE option to append these files.
NOTE: 0 observations added.
NOTE: The data set CLASSLIB.HTWTDATA has 4 observations and 3 variables.
NOTE: Statements not processed because of errors noted above.
NOTE: PROCEDURE APPEND used:
      real time           0.11 seconds
      cpu time            0.11 seconds

NOTE: The SAS System stopped processing this step because of errors.