If you export the data files to csv you can edit them (except in Excel 2003 which screws up the format) and use my editor program available at
http://www.bourtonville.co.uk/OOTPEditor/publish.htm to reload the .csv into the parks.dat file.
You select your game directory at the top, and then on the Parks tab, click Start and Load and it will re-read in the parks.csv - it doesn't allow for deleting of parks though.
If you only have Excel 2003 you'll probably need to go into the file in WordPad and make sure that each line has the requisite number of columns (having just had a look at parks.csv you might get away with it, but if it doesn't import edit the file in wordpad or a different version of Excel)