this is easy to do if you use cpanel/WHM
first on your site you would list the plans as normal with the addon domains displayed and a small footer toi the plans saying addon domains are activated with justification.
then when a client requests these and you agree, you go into your root WHM >> list accounts and then find the account of the client and then mouse over the clients username and the edit button will appear under the username click on this and it will open an edit are for than account. then you just add the addons and then click save.
this is then done