Since I play on a modded mod pack. There is a chair in my claims that I want other people to sit on it too.

But when interacting to the chair, it prompts this message:

This option that I saw makes me think that there could a way to add a block ? If it is possible... how?

@thexaero

In the main config file openpartiesandclaims-server.toml, you can add blocks to exception groups or create new ones altogether. The group "Controls" is in the config file by default.