Appending Data with a Data Step
This makes an unnecessary pass through both data sets. Also note the number of variables in the new data set. Is this what you wanted to happen?

data moredata;
   input h w g $;
   datalines;
63 120 F
72 200 M
;
data classlib.htwtdata;
	set classlib.htwtdata moredata;
	run;

SAS Log:

NOTE: There were 4 observations read from the data set CLASSLIB.HTWTDATA.
NOTE: There were 2 observations read from the data set WORK.MOREDATA.
NOTE: The data set CLASSLIB.HTWTDATA has 6 observations and 6 variables.
NOTE: DATA statement used:
      real time           0.16 seconds
      cpu time            0.16 seconds