You can revert Xtreme 3.2 to toggle back to the selector tool with the space bar using the download from Xara's website "How do I reset the spacebar as a short cut to the selector tool?" but I don't think they created anything similar for v4 or v5.

Maybe v4 and v5 use a similar file to define the keyboard shortcuts, you might try tinkering: it looks like the toggle for the selector tool is about four lines from the bottom in the downloadable file for v3.2.