Given a::b.pm:

package a::b;
require Exporter;

our @ISA = qw[ Exporter ];
our @EXPORT = qw[ foo ];
our @EXPORT_OK = qw[ foo bar qux ];
our %EXPORT_TAGS = (
    all => [ @EXPORT_OK ],
);


sub foo{ print 'fooey|' }
sub bar{ print 'Baaa!'  }
sub qux{ print 'Qux?!'  }

1;
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And a::b::c.pm:

package a::b::c;
use a::b qw[ :all ];
require Exporter;

our @ISA = qw[ Exporter ];
our @EXPORT = @a::b::EXPORT;
our @EXPORT_OK = ( @a::b::EXPORT_OK, 'sensible' );
our %EXPORT_TAGS = %a::b::EXPORT_TAGS;

push @{ $EXPORT_TAGS{ all } }, 'sensible';

sub sensible{ print 'sensible' }

1;
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Then importer.pl:

#! perl -slw
use a::b::c qw[ :all ];

foo();
bar();
qux();
sensible();
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Produces this:

c:\test>importer
fooey|
Baaa!
Qux?!
sensible
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Anything added to a::b.pm will automatically be available for import through a::b::c.pm without modifications.