Hello there,

In accounting -> Configuration -> Taxes -> Taxes, I edit a tax.

I click to add a Children/Sub Taxe to this edited taxe. A new window appears :

At this place, we have to imperatively create a new tax. Impossible to select an already existing taxe in the Tax Name textbox... 

The only way I have found to add an existing child taxe to a taxe is directly in the database (pgadmin), in the model account.tax. There, I modify the column parent_id manually.

Thanks to help.