The exact location of the file you need to copy depends on your OS. For me, running Windows XP, it's in:
C:\Documents and Settings\<USER>\Local Settings\Application Data\CCP\EVE\c_program_files_ccp_eve_tranquility\settings
<USER> is the windows user name.
The file containing the quickbar is core_user_*.dat
for example core_user_9999999.dat
* is a number (it's a 7 digit number on all of my accounts). I don't know how CCP comes up with the number but it's some kind of account ID. For my 6 accounts the smallest ID number is the first account created. The larger the number the later the account was created. You can figure out which file is for which account as long as you know the order the accounts were created in.
Find the file with the settings you want and copy it to the files for the accounts you want the same settings on.
Don't forget doing this will change a lot of other settings too, not just the quickbar.