Step by step:

1. Access SysConfig -> Ticket -> Core::Ticket
2. Add a new value on Ticket::ViewableLocks parameter. This value must be 'lock'
3. Make a copy of /opt/otrs/Kernel/Modules/AgentTicketQueue.pm to /opt/otrs/Custom/Kernel/Modules/AgentTicketQueue.pm (you will need probably to create the folders Kernel and Modules inside Custom folder)
4. Open the file /opt/otrs/Custom/Kernel/Modules/AgentTicketQueue.pm on your prefered editor and change the code bellow:

 

my %Filters = (
All => {
Name => 'All tickets',
Prio => 1000,
Search => {
StateIDs => \@ViewableStateIDs,
QueueIDs => \@ViewableQueueIDs,
%Sort,
Permission => $Permission,
UserID => $Self->{UserID},
},
},
Unlocked => {
Name => 'Available tickets',
Prio => 1001,
Search => {
LockIDs => \@ViewableLockIDs,
StateIDs => \@ViewableStateIDs,
QueueIDs => \@ViewableQueueIDs,
%Sort,
Permission => $Permission,
UserID => $Self->{UserID},
},
},
);


to


my @LockedIDs = (1,3);
my %Filters = (
All => {
Name => 'All tickets',
Prio => 1000,
Search => {
StateIDs => \@ViewableStateIDs,
QueueIDs => \@ViewableQueueIDs,
%Sort,
Permission => $Permission,
UserID => $Self->{UserID},
},
},
Unlocked => {
Name => 'Available tickets',
Prio => 1001,
Search => {
LockIDs => \@LockedIDs,
StateIDs => \@ViewableStateIDs,
QueueIDs => \@ViewableQueueIDs,
%Sort,
Permission => $Permission,
UserID => $Self->{UserID},
},
},
);