Upgrading after domain registration only

[Pat_lh]

Hi All,

I am setting up a new whmcs node and i am stumbling over the following:

fitst of all everything works as expected, i don't have any issues, technically speaking. However

if a client only registers a domain name then that is handled accordingly.
o problem there. But if that same user decides to take hosting later on, there is no way he can do that (at least not that i can think off)
Also, there is nothing handed over to plesk either, so de DNS isnt active either.
Is there a way to acocmplish this ? 

What i really want is some kind of trigger then a client orders a registration only (without hosting) that this is triggered upon registration so i can have DNS setup and working in plesk 
and the user needs to be able to upgrade his registration only with hosting.

Thank you in advance for answering.

Regards,

Pat