You can always use an Insert command to insert a part for the active assembly. For more information, see:

Under Prediction in the Options window, Show empty row in System file Parts Table controls whether a blank record is shown in the Parts Table. When this checkbox is selected (default), a blank record is shown. In this case, you insert a new part by entering data in this blank record. For more information, see Inserting a Part Using a Blank Table Record.

If, however, Show empty row in System file Parts Table is cleared, the last row in the Parts Table displays the < Click here to insert a new record > command. Clicking this command opens the Insert Prediction Part window, which you complete to insert a new part. For more information, refer to Inserting a Part Using the Insert Prediction Part Window.