What you are trying to do is a little unusual - adding an Excel 2007 feature in an Excel 97 (HSSF/BIFF8/.xls) file. Having said that, Excel *can* do this with a little coaxing, and POI *will* need to handle post-BIFF8 features within XSSF.
So far, POI doesn't support much more than the standard 'built-in' functions of Excel 97. That's why you are getting the formula parser error. There is no way around formula parsing - the Ptg tokens also need to be encoded somehow (and in XSSF, formula parsing is still required as a syntax check). It seems like Excel encodes SUMIFS using a NamePtg that points to a defined name called "_xlfn.sumifs". This technique is different again to that used for 'add-in' functions (which POI partially supports). To make this all work, POI needs to: - recognise this category of function and get the metadata (needed for operand class transformation of parameters) - find or add the appropriate DEFINEDNAME record (seems like some extra flags (0x380) are required) Some thought would also need to be given (in the HSSF world) to version issues. Even though Excel 2007 can save BIFF8 (.xls) files containing SUMIFS(), it does so with warnings. Apparently Excel 97 will fail to evaluate those formulas. If evaluation (by POI) is required, the formula evaluator will need changing too. New contributors and patches are always welcome.
