Need help with Role Provisioning and Maintenance of Custom Roles

Summary

Need help with Role Provisioning and Maintenance of Custom Roles

Content

We have a third party application through which we want to manage user access provisioning in cloud.
We can get the roles assigned to users, but we don't have any possible way to assign data access to user after the role is assigned.
As an example if user needs Accounts Payable Manager for BU1. We are able to assign Accounts Payables Manager role through an API, but no API is available for assigning BU1 data access.

One option that is being considered is create a Custom roles by assigning data security profile of respective BU, something like