according to the docs: http://docs.moodle.org/23/en/My_Moodle#Adding_a_block_to_the_My_home_page_for_all_users
Adding a block to the My home page for all users

An administrator can add a new block to the My home page for all existing users (not only new users) as follows:
Turn editing on for the front page
Add the block to the front page
Edit where the block appears and set the page contexts to 'Display throughout the entire site'
Go to your My home page and again edit where the block appears and set 'Display on page types' to 'My home page'

I suggest when adding something new on the "Default My Moodle page" you have in the settings of the block the option to add it for all users or for new users only