collect IDOCs

Hi experts,

I would like to know outbound IDOC to file interface.

Here, i have collect same data type of IDOC ,for example collect one day IDOCs and trigger at night , so that collect all information and create to single file .

Otherwise,can we trigger IDOC and update the same file ???

If i am choosing choosing collect IDOCs in we20, still it is creating single messages in PI but i need to be created one message with multiple IDOCs.

Regards,

Deva