"Fellow Member" is not really a recognised term - someone is either a Member or is a Fellow (though the website does refer to it as Fellow membership to promote confusion). Election to Fellow has specified criteria (details available on the website) but having professional registration is not one of them. It is therefore possible to be elected to FIET without having IEng or CEng.
Note here the word 'possible'. I would add that while it is possible, the candidate for Fellowship is expected to show commitment to the industry and professional achievements, so if a candidate is suitable for registration and has not applied it would raise the question as to why not. Of course having applied but still waiting for an outcome would probably be an acceptable answer to that.
