Start the transaction using transaction code KAH2

Double-click 

Enter the group in which the cost element must be included

and 

Click the group in which you want to include a new cost element

and then click 

you can click several times if you want to include more than 5 cost elements

Enter the code of the cost element you want to include 

then  and 

There is a message at the bottom of the page 

You can leave the transaction