DDE is not implemented in fpspreadsheet. I thought this is a Windows-only technology, and to be honest I don't even know how it works internally, and I have never used it in my own Excel files. But of course you can try to implement the formula on your own and upload a patch (formula support is in unit fpsfunc).
But at least you should prepare a simple file with such a DDE formula and upload it here for me (rename its extension to zip to bypass the forum software). FPSpreadsheet should not crash when that unknown formula is detected, and I can try to fix it although I will not implement the formula.