Hi, @Hector Huerta Tricolor Welcome to the Microsoft Q&A Platform, and thanks for your query.
You cannot grant any existing roles. Here is the documentation page on data-plane access (read or write from a database account):
That doc walks you through the process that you can follow using Azure CLI or PowerShell to grant a principal (identity) access to read or write to resources in Azure Cosmos DB.
Please let me know if you need any additional information.
Regards
Geetha