I’ve created Extension attribute for the gross sales order

I’ve under knowledge to set

Array
(
    [a] => test1
    [b] => test2
)
Array
(
    [a] => test3
    [b] => test4
)
Array
(
    [a] => test5
    [b] => test6
)

I wish to set these all array in single extension attribute title custom_attribute like this

"custom_attribute": {
      {
        "a" : "test1",
        "b" : "test2"
      },
      {
        "a" : "test4",
        "b" : "test5"
      },
      {
        "a" : "test5",
        "b" : "test6"
      }
    },

at the moment my code takes the one final array.

How can I achive this please share any reference or assist into this.