> my tax app won't read it as the Macbook classifies it as keynote
The MacBook (or, more specifically, the Finder) does classify files based on their filename extension, and so a filename ending in .key would be identified as a Keynote presentation file by default.
However, that has no bearing on whether your tax app can open or read the file. Typically you would do this within the app itself (maybe via File -> Open, or File -> Import, or some such). In this case, the Open File dialog shouldn't differentiate between file types and should allow the file to open.
Failing that you may be able to drag the .key file to the application's icon in the Finder, or Ctrl-click on the file and choose Open With... from the popup menu and select the target app.
Much of this does depend on the way the application developer has written their apps to identify files, but the fact that the Finder identifies the file as something different shouldn't make a difference.