Heya kamrann! The proper api exposure for these classes has not yet been determined, but one thing you can do is:
In the GetActions function of your custom subclass of IAssetTypeActions (lets call it FFooActions for asset type UFooAsset), you can get the IAssetTypeActions of the engine-level asset that UFooAsset extends by using IAssetTools::GetAssetTypeActionsForClass() and passing in the parent class of UFooAsset.
Just call GetActions on the engine-level IAssetTypeActions before or after adding your own (which will determine menu order).
It would look like this:
void FFooActions::GetActions(const TArray<UObject*>& InObjects, FMenuBuilder& MenuBuilder)
{
FAssetToolsModule& AssetToolsModule = FModuleManager::LoadModuleChecked<FAssetToolsModule>(TEXT(“AssetTools”));
TSharedPtr<IAssetTypeActions> EngineActions = AssetToolsModule.Get().GetAssetTypeActionsForClass(GetSupportedClass()->GetSuperClass()).Pin();
if (EngineActions.IsValid())
{
EngineActions->GetActions(InObjects, MenuBuilder);
}
// Add more actions here
}