Salesforce Flow Orchestration is designed to manage complex, multi-step business processes that span multiple departments and systems. Here's how it can be structured to meet the requirement:
Automate Multi-Departmental Process:
Use Flow Orchestration to define and manage the steps involved in the multi-departmental process. It allows you to break down the process into stages and define the sequence of actions and approvals required.
Capture Outcome in Salesforce:
After completing the external process, Flow Orchestration can be configured to update Salesforce records with the outcome. This ensures that the results of the automated process are reflected within Salesforce.
Orchestration Capabilities:
Salesforce Flow Orchestration provides features such as task assignments, decision elements, and complex branching logic, which are ideal for managing multi-departmental workflows.
References:
Salesforce Flow Orchestration Documentation

MuleSoft Composer is designed for business users to create integrations without deep technical knowledge. It is ideal for scenarios that involve:
Data Transformation and Integration:
Non-technical users can easily connect different systems, automate data transfers, and transform data from one system to another using a no-code interface.
MuleSoft Composer provides pre-built connectors and an intuitive interface to set up these integrations.
Simplified Automation:
The platform is optimized for creating straightforward, rule-based automations where complex coding is not required.
Therefore, automating data transformation tasks by a non-technical user fits perfectly with MuleSoft Composer's capabilities.
References:
MuleSoft Composer Documentation

Develop System API for Loyalty System:The first step is to develop a new System API that integrates with the Loyalty system. This API will handle communication with the external system via the provided HTTP endpoint.
Reference:MuleSoft API-led Connectivity
Utilize Existing Customer System API:Use the existing System API for customer data to retrieve necessary customer information. Combining these APIs ensures a modular approach and reuse of existing assets.
Reference:API-led Connectivity
Create Process API:Develop a Process API that combines data from both the Customer and Loyalty System APIs. This API will process the data, apply business logic to filter the top 10 percent of loyalty point holders, and format the results.
Reference:Designing Process APIs
Create Experience API:Develop an Experience API to serve the business consumers. This API will provide a user-friendly interface for initiating the integration and retrieving the results as a CSV file.
Reference:API Experience Layer