Sub Items can be imported to dRofus as subs to Items. 

Read about: Introduction: Import/update with Excel-file

Under 'General' 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'.