you certainly can in A2003.
Have you tried it? Can you insert rows? Can you move rows?
This is one of the best threads I've found on the specifics. I'll add here what I've compiled on the matter for A2003. Hope I don't retread too much old ground.
utteraccess.com/forum/Edit-Import-specification-t1950181.html gives an overview expressing that it's a huge pain to edit specs in 2003.
skrol29.com/us/vtools.php purports to address this; I haven't run it. Freeware. He leaves source code viewable; he has an extensive amount of code dealing with specs in Access 03. See the routines beginning with p_Spec_.
CurrentProject.ImportExportSpecifications.Add name_of_spec, xml is demonstrated in mrexcel.com/forum/showthread.php?t=615680 where it purports to build an import spec, supposedly for A2007 and later.
You might try working with schema.ini (use with TransferText) as detailed in support.microsoft.com/default.aspx?scid=kb;en-us;155512 Note it is only "certified" for Access 97 (but I can't imagine it's not still usable).
The closest I've [seemed to] come to inserting lines is appending sufficient 'dummy' lines at the bottom, Ctrl-A, paste to Excel, modify, Ctrl-A, Ctrl-V back to Access. This seems to work, but most gruesomely. You get 5 error message boxes that you bite your lip and ignore; and maybe have to delete a duplicated first record field; and delete an autonumber key field that the Easter bunny puts in the created table. This sounds absurd, I know; but once you get confidence in it, it's awfully nice to make changes in Excel vs. the clunker interface (unsizable, too, gee thanks) in the Access wizard.
Not a slam dunk in any way. Galaxiom, FYI I also roll my own, with code using tableDef and .CreateField, .Size and .Type.
Sorry for lame links; you need 10 posts on this board to enter formed URLS.