Try an update query... I created a small database with five records in one column and an empty column waiting for the converted output. ImportData was the column with the data and ConvertData was the empty 'receptacle'. On the Query tab I designed a new query, added my table, clicked on both fields to add them to the design grid, then selected "UPDATE QUERY" from the Query menu item. On the third line of the design grid under ConvertData I typed:
=Mid([importdata],5,2) & "/" & Right([importdata],2) & "/" & Left([importdata],4)
When I ran the query it converted entries like 19990807 under ImportData to 08/07/1999 under ConvertData.