How do I dynamically insert or remove 'assignee' to Sequential Multi-Instance?

Hello ,
I got a troble on Sequential Multi-Instance for dynamically insert or delete ‘Users’,
For Parallel Multi-instance i used addMultiInstanceExecution/deleteMultiInstanceExecution to insert or remove users,
But it looks like doesn’t work to SequentialMultiInstance… the user wasn’t be added by the interface ‘addMultiInstanceExecution’.
So , What shoud i do ? please help me to check this issue … thanks.

Best regards