It's possible to add this relationship: https://musicbrainz.org/edit/48071031

But it is essentially impossible to edit or remove it:

https://musicbrainz.org/instrument/63021302-86cd-4aee-80df-2270d54f4978/edit is forbidden to normal users.

https://musicbrainz.org/artist/d74227f7-e3eb-4967-a4f5-db383e383992/edit does not provide access to the relationship because of the cardinality changes implemented several years ago, IIRC.