Hi Alexis,
The standard way of doing the activity is by adding the same manually, there is no other way. Every time you have a new G/L Account created and requires revaluation then add the same manually in OBA1. Client will have to raise an support ticket to add the G/L account in OBA1.
Please check with ABAPer if they got some solution to the same but that would not be an standard way of doing the activity.
Regards,
Vishal