Sub Items can be imported to dRofus as subs to Items.
In the Items Module, select the Import/ Export Tab→ Import Sub Items
In the dialogue window, choose an Excel File. Read about: Introduction: Import/update with Excel-file
...
Under 'General' Choose select the Unique Identifier for the Parent Item. This key can be Item Number, Name, BIM ID, or Reference and map the column. Repeat this step for the Sub Item. Map the quantity column and Calculate quantity field to the relevant columns in the excel file. Then select 'Import'.
...