How to create an extra row after the last row of each BY group?

[somebody]

I have a stock dataset. I would like to create 1 extra row, which is the date, after the last date of the stock in the dataset. So for example, my dataset contains prices for stock IBM from 20190102 until 20190630. I want to create 1 extra row for the date 20190701.  One extra requirement is that it cannot be a holiday, i.e. It has to be a business date.

I have a dataset called TRADINGDAYS that has 1 column of dates that are business dates. it looks like this. The dataset is attached

Begin 
20190102
20190103
......
20191230

[PeterClemmensen]

The easy part can be solved like this.

Post your tradingdays data if you want that included as well

data want;
   do until (last.stock);
      set sashelp.stocks;
      by stock;
      output;
   end;
   date + 1;
   output;
run;

[somebody]

Thanks. I edited my question and attached the tradingdays dataset. can you please have a look and incorporate that ?

[PeterClemmensen]

Ok, just needed the structure. Here, I created a very simple replica of your tradingdays data set. It is just the day after the last date for each atock in sashelp.stocks. 

See if it works for you

data tradingdays;
input begin :ddmmyy10.;
format begin ddmmyy10.;
datalines;
02/08/1986
run;


data want(drop=begin);

   dcl hash h(dataset : "tradingdays(rename=begin=date)");
   h.definekey("date");
   h.definedone();

   do until (last.stock);
      set sashelp.stocks;
      by stock;
      output;
   end;

   do until (h.check());
      date + 1;
   end;
   
   output;
run;