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This would change all tabs to ordinary spaces. You can actually just type a tab and space character, but I used the hex notation so you see what you're doing. The space could be any other character(s) you desire, or no characters at all (make sure the To Text: field is empty).
Hex codes?
Found here on one of my installs:
C:\Program Files (x86)\Adobe\FrameMaker9\Documents\Character_Sets.pdf