I'm not aware of any way do such a substitution while performing a Text Import. However you can change the Text Import preferences to make "spaces" (2 or more) be the only field delimiter or "spaces" can be added to an existing list of delimiters also specified in Preferences.
Preferences>Text Data Files>Import Settings
This can be scripted though not documented (at least in 8.0.1), and I am unaware of how, via scripting, to save the existing Import Settings in order to restore them using scripting after changing and using the the changed settings.
JMP support confirms that it's safe to use this scripting approach.