Today, once a CTA has been created as a Company CTA, you cannot add a Relationship to it, even if that CTA Type is available for that particular relationship.

This means, that as a user, if you created a CTA and forgot that step, or you want to retro-actively add it due to a new workflow, you would have to delete that original CTA and create a new one. This is not idea, because 1) that is an obscene amount of extra work to add a value to a field 2) You would lose out on the original metadata including, but not limited to - the original CTA created date, original due date, task completed dates, etc.

If I have an already created CTA, the OOB Relationship field should be editable as long as that CTA type is available for that Relationship type. 

Even creating secondary, duplicate lookups is a waste of time because then you have two places to enter in the same data and it gets to be a confusing mess. The fact that we cannot change this makes for a poor use experience and a very inflexible design that doesn’t allow for admins to cater to complex and/or evolving business needs.