Although an upgrade application will look for a previous version or 'qualifying product' during the installation process, it will bring you to a window where you can enter a license key for the 'qualifying product.'
That is, when you install FileMaker Pro 7 Upgrade, you will be prompted to enter the FileMaker Pro 7 license key. Next, the installation will look for a previous version. If it can not find a previous version or 'qualifying product,' the installation will tell you that it could not find a qualifying product and you should enter a qualifying product's license key.
Try using the FileMaker Pro 5.5 key here. It is not required to have a previous version installed on the same machine in order to install the upgrade application.
I also wanted to give you a heads up that, while you are upgrading, FileMaker Pro 7 is no longer a supported product. Hopefully, FileMaker Pro 7 will meet all of your solution's needs. If you are interested, you can try a free 30 day trial of FileMaker Pro 9 here: