No ITR for 2017-18 cannot be filed after 31 March 2018. if ITR is previously filed then you can revise that ITR otherwise you cannot file new ITR for 2017-18
In normal provisions no way for filing ITR AY 17-18...
But, You can try file Condonation delay request for filing ITR AY 17-18 to CIT / ITO... In case the request is accepted then You can file Your ITR based on the request...