Hi Nabil,

When you buy third party modules you can just add them in a custom directory and add this custom directory in your Odoo configuration file. Thanks to adding it there your Odoo will be able to find all the custom modules you've added and you can just install them on top of the community version of Odoo. Are you talking about the accounting related modules that are in the free community version or are you talking about the more advanced accounting which is in the enterprise version? If you're talking about the community version you already have the apps available. If you're talking about enterprise apps there is only one way and that is buying Odoo enterprise licenses along with the apps. In this case you can then add the enterprise modules and install it. If you're talking about a third party accounting app you can add it to the custom modules and install it.

Every module that is added to your Odoo instance (under your own folder) will have the source code available so you can view and edit it. Make sure that the app is under a license which allows modifications though.

Regards,

Yenthe