PO & SM rôle in nexus

All the rules and guidance of Scrum as it's defined in the Scrum Guide continue to apply to Nexus.

A Nexus is a group of three to nine Scrum Teams plus a Nexus Integration Team that consists of the Product Owner, a Scrum Master, and Nexus Integration Team Members who are often Scrum Team members from the various Scrum Teams that create the Nexus.

Because a Nexus is centered around a single product, there is only one Product Owner across all the teams in the Nexus. Although the Product Owner could still participate as a Developer, I would find it highly unlikely that it would be effective. The amount of time needed to work with and across each Scrum Team to create, communicate, and maintain the Product Backlog would increase to the point where a Product Owner could not carry out product management and Product Backlog management activities and have the necessary focus to be a Developer. The Product Owner would be part of the Nexus Integration Team, as defined by the Nexus Guide. There would be nothing to stop the Product Owner from also acting as a Developer for one of the Scrum Teams and participating in the various events, such as the Nexus Daily Scrum or a Scrum Team's Daily Scrum, as a Developer.

The Nexus Guide is silent on how many Scrum Masters exist. Each team does have a Scrum Master. There's nothing prohibiting a Scrum Team's Scrum Master from also being a Developer. In fact, there's nothing that expressly prohibits a Scrum Master from also being the Product Owner, but it's commonly seen as something to avoid. The Nexus Integration Team also has a Scrum Master. This person may be a Scrum Master from one of the Scrum Teams in the Nexus or it could be a dedicated Scrum Master. This is really up to organizational design. In my experience, a person dedicated to the Scrum Master role could work with up to 3-4 teams (depending on the experience and self-management of each of those teams) at once. A Nexus could have 0 full-time Scrum Masters, which would mean that each Scrum Team's Scrum Master is also participating in another role. A Nexus could also have N Scrum Masters, where N is the number of Scrum Teams in the Nexus, where it is likely (but not mandatory) that one of the Scrum Masters would also serve as the Scrum Master for the Nexus Integration Team. A Nexus could have N+1 Scrum Masters, where N is the number of Scrum Teams in the Nexus, where the Nexus Integration Team would have its own dedicated Scrum Master. The Nexus could have 1 Scrum Master, where the Scrum Master is the Scrum Master for all the teams and the Nexus Integration Team. The Nexus could also have between 2 and N-1 Scrum Masters, where the Scrum Masters divide up the various coaching and facilitation needs of each of the teams, including the Nexus Integration Team.

no indication if the Po and the scrum master in the nexus can play the role of developer?

In the Nexus yes, but not in the Nexus Integration team, since that team has no Developers.

Are the PO and SM can be a part of Nexus intégration team in addition to their role? 

Not just in addition, this accountability has precedence. Integration comes first.