This can't be done with just ldapmodify, but you can string a couple things together to do what you're asking. The following runs ldapmodify against an ldif file that we build on the fly from an ldapsearch plus the desired modifications.
ldapmodify -f <(ldapsearch -LLL -b ou=People,dc=example,dc=com -s one uid=* dn | sed 's/^$/add: customerNumber\ncustomerNumber: 12345\n-\nadd: accountNumber\naccountNumber: 12345\n-\n/' )
Note: This is gives all matched entries the same customerNumber and accountNumber. If that's not what you want you'll have to use something more complicated than
perl. Also, it's probably complicated enough that you'll want to do it in more than one line first. Make the ldif, review, then run ldapmodify.