Nominating owner of a bpm task by random ?

Hi`

I need help to implement my customer's request with BPM.

I am implementing a process with Netweaver PO 7.31 SP6.

And, my customer want to assign the owner of a task  automatically by random in a pool of possible employees (not potential owners).

For example,

Task "Approval" has 5 possible employees with employee id 10001, 1002, 1003, 1004, 1005.

When the process instance arrives at task "Approval",

the Process should assign one of  5 possible employees by random as the task owner automatically.

for example, let's assume employee 1002 is assigned in this process instance.

When another process instance arrives at task "Approval",

the task owner should be one of 4 possible employee except employee 1002 by random.

Do you have any idea ?

Please help me ;;;