After struggle a lot, I found a solution and, now, I can answer my own question.
The best way to implement a solution to the question above is to implement a 'group by' step.
1. Create a sequence step starting in zero and increased by 1.
2. Create a caculator step:
2.1. Set a constant 'group_size' with the number of rows needed to be grouped (1000 in this example). Set the type as Integer.
2.2. Set a variable (I named 'batch_group') by dividing the sequence (field A) per 'group_size' (field B). Set the type as Integer.
3. Create a 'group by' step:
3.1. Set the group field as 'batch_group'.
3.2. Set one aggregate with a name, like 'body_batch', informing the column to group in subject, and type 'Concatenate string separated by ,'.
With these steps, the output would be grouped by the value of 'batch_group', and all values will be concatenated in the column named in step 3.2, separated by comma.