Welcome to community, @LucasPellizon
In this case, you could use Javascript in advanced settings to import the inherited custom field - copy field “Client” values from Epic to Children.
https://docs.eazybi.com/eazybi/data-import/data-from-jira/jira-custom-fields/javascript-calculated-custom-fields#JavaScriptcalculatedcustomfields-Inheritedfielddimension
Just be careful with the Javascript functions, as some custom field types require .value to return the right. You can validate the script (before using it in advanced settings) for one issue key
See similar discussion here: Filtering on Program Increment at Epic Link level - #2 by zane.baranovska
Then you could use the new “Epic Client” field in your reports with measure “Hours spent” and all hours would be grouped by the client of the epic.
But this approach would work only with single-select Jira custom fields (one value per field)
Martins / eazyBI support