Adding 'Select' Field to existing LINQ Query

[jsurpless]

Hi

I know that it's possible to build upon a query using .Where, .Group, etc... but can I add a field?

For instance, here's my LINQ

Dim DatabaseComponent_Table_Query = From ADG_Instance_Drawing In DatabaseComponent_Table.AsEnumerable() _
                        Select AutoCADReferenceAttribute = ADG_Instance_Drawing.Field(Of Object)(c_Database.ReferenceAttribute_Field), _
                               Instance_Drawing = ADG_Instance_Drawing.Field(Of Object)(c_Database.InstanceDrawing_Field), _
                               AutoCAD_Block = ADG_Instance_Drawing.Field(Of Object)(c_Database.AutoCADBlock_Field), _
                               AutoCAD_Block_XCoord = ADG_Instance_Drawing.Field(Of Object)(c_Database.XCoordinate_Field), _
                               AutoCAD_Block_YCoord = ADG_Instance_Drawing.Field(Of Object)(c_Database.YCoordinate_Field), _
                               AutoCAD_Block_ZCoord = ADG_Instance_Drawing.Field(Of Object)(c_Database.ZCoordinate_Field) _
                        Where Instance_Drawing = strAutoCADFile AndAlso AutoCAD_Block IsNot Nothing

The last Selected field (AutoCAD_Block_ZCoord), I only want to select if that database is provided (it's optional)... I realize that I can define two separate queries with an If statement on that field but I was wondering if I can append that field somehow... I tried looking through the examples but I'm not quite sure what I"m looking for...

Thanks!