Solved... It was really simple. Thank you for your time ;)
I add the key into the keys in ItemUpdating event in DetailView.
protected void dtlvUserDetail_ItemUpdating(object sender, DetailsViewUpdateEventArgs e)
{
DropDownList ddlEditPermissions= (DropDownList)dtlvUzivatel.FindControl("ddlEditPermissions");
e.Keys["permission"] = ddlEditPermissions.SelectedValue;
}