You can extend a dataset by adding variables from another dataset that match the current one on some key variable (typically called a ‘left join’).
To append data, click the dataset name in the left Folder Sidebar, make sure you have editing permissions for it (shown by an unlocked icon in the top right), and then select the Properties option from the drop-down list:
Click the 'Join' tab and then select a dataset to join:
Select the key variables and then click the Join button:
Joining may take some time depending on the size of your datasets. Once you are finished, you will either see a success message or you will be informed that the join failed:
Check that the keys are the same type and actually identify rows that should match, and ensure that the new data you’re joining does not contain any aliases already present in the dataset. The joined data will appear in a new folder in the list of variables.