Are you the printer? If not, did he ask you to do this?
Imposition is the printer's responsibility and you should not be sending an imposed file to print.
Yes I am.
I got the indd file and cannot create the A3 print of the A6 brochure...
Can you help me how to do this in indesign? Or some other software?
You don't have some sort of imposition software that you normally use?
Print Booklet has a Perfect Bound option that can give you 4-up pages (8 page signatures), but it isn't clear if that's waht you want, or if you want two 2-up pages that are the same that you will split. We need a lot more info, and you need to understand that Print Booklet is not designed for professional impostion. It's there to allow users to do simple booklets on their office printers.