How to deal with external suppliers with a Scrum approach?

If those changes to an external system are required for a Done increment of work, then the Developers would need to ensure that this work happens. They are the ones who will be making the commitment.

Perhaps the supplier shares in this accountability, and ought to provide some of the Developers as well.

One of the Developers is probably the best choice of person to interact with external suppliers, especially since they have the knowledge of what is needed or what would be an acceptable solution. The bigger questions are probably around how the Scrum Team works with external suppliers, since they probably aren't operating on the same cadences or even with the same vision as the Scrum Team.

I see what you mean Thomas and Ian, but couldn't be too much time consumption for a developer to have to deal with the external vendors, particulary when as mentioned they don't have the same cadences or vision of work? 

My concern is them spending too much time in work that will not be visibile in the Sprint Backlog, therefore affecting velocity without any reason visible to the organization.

couldn't be too much time consumption for a developer to have to deal with the external vendors

The Developers are accountable for the work they do and for the commitments they make.

Perhaps they need to take less work on from the Product Backlog each Sprint, so they can be sure of meeting their accountabilities.

My concern is them spending too much time in work that will not be visibile in the Sprint Backlog

Why wouldn't such a critical part of their plan be visible in the Sprint Backlog?

therefore affecting velocity without any reason visible to the organization.

Why would the organization care about velocity in the first place? Might they be confusing it with value?