BZ,
Could you tell what it's important to use typedlist here, because as I see, the EntityCollection and Fields on Related Fields fits better here.
Anyway, If it would be necessary, I would create two typedList in LLBLGen Designer, one for Bill and one for BillLine. Then at BillTypedList, at Custom Code, create a variable and custom property that store a BillLineTypeList. Then you outside the typedlist fill the BillTypedList and its BillLineTypedList property.
Are you using Adapter or SelfServicing?