Hi @PedroPablo_Leon!
Unfortunately, both these actions aren’t available with Boards, it is possible to convert a task into a subtask in a List project (Convert task to subtask - #2 by Aisling_Grogan) but the same action isn’t possible with Boards.
Multi-selecting is already available for List projects (Task actions: create, move, duplicate, and more | Product guide • Asana Product Guide) but not for Board projects yet. I would highly encourage you for this feature request on this thread: Task actions: create, move, duplicate, and more | Product guide • Asana Product Guide
My apologies for not being of a greater help here! If there is anything else I can help with, please let me know!