Like any other variable, in regard to YAML you can use square brackets in gitlab ci yaml file.
For the sake of your shell, you should not however use variable names with dots and square brackets in them. But you can easily send them encoded as YAML for example:
variables:
FOO: "A bar and a variable enter the global scope..."
yaml_bag: |
sec.access.clients[0].username: svc_fkl_01
sec.access.clients[0].password: abrakadabra
sec.access.clients[0].roles: FKL
sec.access.clients[1].username: svc_dp_01
Sure. What have you tried? What constitutes sending and what is that service? Which one is the command you use to send variables to that service? What is your specific question?