Probably this will work for you. First, for existing mdf file uncheck check box "Automatically grow file' (i.e. to prohibit it's growing). Then create, let say, 2 extra ndf files. Load into your DB (using DTS for example) copy of your tables (with new names that later you will rename). That's it- new ndf files will be filled with your old data (actually, after that you need to drop initial tables and rename new table copies to old ones).