Sub Accounts for customers

hello is there a feature or any way you can have sub accounts for customers? i have a client who wants one account for a customer but the invoice realtes to over 100 different sites. so they email the same person but they want to select what site it is and this shows on invoice. if there is not a feature is there any way round this?

Thanks

Also not to manually have to do it each time

they are currently on clear books and they can select this from a dropdown

Thanks