Can't you convert to the date to a string and then use expression builder to pull out the appropriate sections of the string into the format you require?
This 'should' return today - 90 days in CCYYMMDD:
(DT_STR,4,1252) DatePart("yyyy",getdate()-90) + (DT_STR,4,1252) DatePart("MM",getdate()-90) + (DT_STR,4,1252) DatePart("DD",getdate()-90)