Answer
If the OBJKEY column doesn't exist on a view, custom fields can't be added to it — this column is deliberately left out of certain views by the relevant product teams when they've determined it could cause complications (for example, a view built as a union of two different Logical Units commonly doesn't get an OBJKEY, per Foundation1 framework recommendations). Adding OBJKEY to such a view yourself would require a genuine customization, and should only be attempted after carefully analyzing whether doing so could cause other issues, since its absence was usually a deliberate design decision rather than an oversight.