Hi there,
Are those fields (LastModifyDate, IsDeleted) are actually database columns for every table?
I think inheritance isn't for this. All depends on what you want to achieve and how is your Database model.
For instance, suppose you have 3 tables: Order, OrderDetail, Customer. All of that tables have columns LasModifyDate and IsDeleted.
When you create your LLBLGen project (DB-first. But you also can create it from model-first) you will have three entities Order, OrderDetail and Customer, all have fields LasModifyDate and IsDeleted. But what is what you want to achieve?
Suppose you want to refactor those common fields, you could create a ValueType with those fields and name it "AuditInfo", then assign that type to refactor the fields in you entities. Now all three entities have a field of type "AuditInfo".
You also could create an Interface in your code and make those entities to implement it. As I said before, all depends on why are you trying to do with this, but I think inheritance is not designed for this scenarios.