How can we improve WHMCS?

Currently, with the API, we are able to create a new user via the AddUser action but there is no way to directly associate the user to an existing client (it can be done via the admin dashboard).

There is an action CreateClientInvite that comes close but it is not suitable for some use cases such as bulk migration of users from another hosting management system to WHMCS.

It is probably not too difficult to achieve that via database queries but it will be good to have an API action for it since we already have actions for DeleteUserClient, GetUserPermissions, and UpdateUserPermissions.