No it's not okay to lie about your salary, even if the motives are understandable, as I believe it forms part of the minimum reference most firms they take from previous employer (others being job title, start date and end date). If they ask you what your existing salary is, just tell them that for confidentiality reasons you are not able to disclose current salary with existing employer - besides that, tell them you'd prefer a salary offer to be made on your credentials and interview performance not on a paypacket.