Currently, in HSSF, there appears to be no way to get the cell number in excel format (eg. C11, GH239 etc.) that would be useful in generating formulas when dynamically creating excel sheets. For this, HSSFCell class seems to be the most natural place to put a method like: public String getCellRefString() {...} that would return a String of the form: AD11 or whatever is appropriate. (Basically convert the cellNum to base 26 (A-Z) and append the row num)
new org.apache.poi.hssf.util.CellReference(row,col).toString()