The short answer is no, you can't. This is a security/immigration issue rather than a Eurostar ticketing issue. You maybe lucky if you try but expect hassle with the authorities and even the possibility of having to wait for the next train to take you to BXS. In a nutshell, getting off at Lille with a BXS ticket creates a Eurostar passenger manifest conflict with various security organisations and also challenges territorial PAF and DGA expectations.