Spreadsheets only serve as templates to prepare data for import either using Atrium Integrator Jobs directly, or by creating Data Management Tool jobs from Data Management Job Console using available job templates (which load data using Atrium Integrator Jobs anyway).

To have a closer look at what each job does, launch Atrium Integrator Console, look for the job you are interested in, then pick "Launch Spoon" icon -- this should start an ETL tool (Pentaho's Kettle - Spoon) which allows you to modify existing jobs (e.g. define additional columns to be loaded from spreadsheets) or make new ones.

Do keep in mind that integration is designed as only the first step leading to changes in final dataset, so predefined jobs leave data in staging forms which may need to be normalized and reconciliated before showing up in final datasets -- jobs dealing with that can be found in normalization and reconciliation consoles of BMC Atrium Core Console.

Regarding sheets in Data Management below may give you more idea

Foundation data mapping - BMC Remedy IT Service Management Suite 8.1 - BMC Documentation

Transactional data mapping - BMC Remedy IT Service Management Suite 8.1 - BMC Documentation

Process setup data mapping - BMC Remedy IT Service Management Suite 8.1 - BMC Documentation

Regarding customization in Data management, there are 2 use cases

1. adding attributes to existing spreadsheets (eg. you have added a new field to CTM:People Form and you want to load data for this field along with other OOB field through Data Management OOB spreadsheet)

2. Create custom spreadsheets to load data to a custom form

To achieve first use case

      a. you need to add new field to staging form used for that spreadsheet(above 3 links can help you to identify the staging form)

      b. add a new column in spreadsheet after OOB column ( you should not change order for OOB columns)

      c. Update atrium job used for that spreadsheet to add mapping for new field (this will load data for this field on staging form)

      d. update workflows to add mapping for new field on staging form( eg. :- CTM:Load People) which pushes record to main form(eg.:- CTM:People)

To achieve second use case:

      a. Create a new spreadsheet in Datamanagement

      b. Design a new atrium job to pick data from your spreadsheet and load that to your custom form (you can get help from any OOB atrium job regarding                    variables used for sheets and Job id etc)

      c. Register new atrium job in data management

      d. Create Job and step template

Below are some links which gives you idea for Data Management customization

Extending UDM Transactional CI Data Load

Adding custom fields to target forms - BMC Remedy IT Service Management Suite 8.1 - BMC Documentation

Managing spreadsheets - BMC Remedy IT Service Management Suite 8.0 - BMC Documentation