Urgent: How to get work list items belongs to other user than current logged in ?

Hello,

I don't use AD but custom security provider to assign destination user in K2, so when I call the work list items by code using c# the K2 set allocated user as current windows authenticated user.

I want to pass something that tell K2 that this (Any other user) is the current user and don't use current logged in user from windows.

help please.

Thanks all.