Yet, there is no graphique way to see Task,

There is Gantt chart.

like a PERT

No, no PERT chart, and I'm afraid we won't have one unless this evolution gets sponsored

with a clear view of the critical path

No critical pathc either, as ProjeQtOr cannot build reverse planning (not yes 😉 )

Gant Chart is nice of course but unless you entered all the ressources and date you can't see anything. And even then if your relations between tasks are a bit complicated it's not clear to see you have forgotten one.
There's one in Project Libre. Since it's an open source project, wouldn't it be possible to add it ?

If it's not for free, would it be possible to have a paying plugging if it's not in the free release ?
If I ask my companie to work on it, would it be possible to sell it as an add in ?

Maybe the forum is not the good place/way to ask this type of question ?
Should I send email ?

There's one in Project Libre. Since it's an open source project, wouldn't it be possible to add it ?

It's asolutely not possible to integrate Project Libre parts in ProjeQtOr (not the same technologies, not the same data structure). But you can export your project in MS-Project xml format and open it in Project Libre (it should be compatible)

if it's not for free, would it be possible to have a paying plugging if it's not in the free release ?

Yes, you may request for a quotation through the contact form.

If I ask my companie to work on it, would it be possible to sell it as an add in ?

It may be possible, but you'll have to deploy it under the AGPL licence of ProjeQtOr (so that your customers may freely redistribute your own code)