Not easily. Your best bet (and I haven't tried this particular thing) is to
try to convert your Excel table into a Word table (e.g. use Edit|Copy in
Excel then Edit|Copy in an empty Word document). At that point you should be
able to see if the non-breaking spaces have been retained. If so, then you
may be able to retain them in a merge (if necessary - perhaps cut/paste into
Word is all you need) by using a trick in your Mail Merge Main document -
normally, to merge a column named "xyz" in the data source, you need a {
MERGEFIELD xyz } field, but if the data source is a Word document, you can
sometimes get away with using a { REF xyz } field or just { xyz }, and if
that works, the formatting applied to the original text is normally
retained.

Peter Jamieson