Is it possible if I know the LLBL relation name to add the relation at runtime? So basically, if there is a relation named OrderDetailEntityUsingOrderID, is there a way to instantiate the relationship in code based on the name?

If so, could you please give a small code snippet...thanks.

I don't know a direct way, but you can use reflection:

System.Reflection.PropertyInfo property =  CustomerEntity.Relations.GetType().GetProperty("OrderEntityUsingCustomerId");
IEntityRelation rel = (IEntityRelation) property.GetValue(CustomerEntity.Relations, null);

In v2.5: List<IEntityRelation> relations = entity.GetAllRelations();

then loop through the IEntityRelation objects and find the IEntityRelation which has the property 'MappedFieldName' set to the name you're looking for.