Excel Drop down list

I am trying to figure out a way to copy subject descriptions from one excel workbook into another that has a drop down list. Some of the subject descriptions would already be in the drop down list however others will not be. Currently I can only copy items that are already in the drop down list. If the list of subject descriptions does not match it will not allow me to copy them in or sometimes it will copy but overwrite the drop down list I have already created. Is there a way to accomplish this task?