for that I temporary created Flexi Task with the names of doers for choices and than save choice in workflow variable.
However, the minus is:
(1) In Flexi Task with branches named (by doers), I should copy and repeat the same actions in every branch;
(2) If I set option "remove branches" in Flexi Task, in this case I cannot get the Doer ID (to send him message for example) because I only have text variable with doer name (text).
That should be easy way to do it. However, I am new in Nintex. Thanks in advice!