"foe" is a rather strong term.
How about changing it to "Ignore"?
That more accurately describes its purpose and function.
It suggests a gentler (yet still firm), rather than aggressive attitude to the Ignored individual.
Often, when "prickliness" occurs, the foe list is mentioned. It might be more intuitive/welcome if its name reflects its purpose.
Why are some individuals excluded from being added to the foe/ignore list?
Specifically, the error message is:
Note that the individual I was trying to add, does not have either admin or mod listed in their Profile.You cannot add administrators and moderators to your foes list.
Though I see no downside to being able to add anybody to one's foe/ignore list.
If any of the Mods is a member of a major distributed Killfile org, they can look up ZS and see detailed discussions about this particular "feature"/bug.