Add all the user from LDAP and add an user to a project

Is it possible to add all the user from the ldap without have to connect independently ?

Export ldap user to csv format, with format compatible with ProjeQtOr import.
Import users from csv file.
NB : to get an idea of expected format, you can export to csv existing users : this format is importable into projeqtor.

How can I add an user on a project ?

From project view, you can only allocate Resources or Constacts, using corresponding table (pay attention, they are both)
So, either you make your user a resource to have it appear in resource list (just check is resource on user screen) or add allocation to project from user screen.