This paper investigates the effect of voice transformation on automatic speaker recognition system performance. We focus on increasing the impostor acceptance rate, by modifying the voice of an impostor in order to target a specific speaker. This paper is based on the following idea: in several applications and particularly in forensic situations, it is reasonable to think that some organizations have a knowledge on the speaker recognition method used and could impersonate a given, well known speaker. This paper presents some experiments based on NIST SRE 2005 protocol and a simple impostor voice transformation method. The results show that this simple voice transformation allows a drastic increase of the false acceptance rate, without a degradation of the natural aspect of the voice.