Ernie-
1. When you USE a table, the DBC is opened for you (if it's not already open)
2. I'm using 5.0a. I just tried what I think you're doing, and it worked fine:

use main!port
index on portno tag goobers
modi struc && The index tag is there
close all
open data main
vali data && No errors

3. The additive clause has no meaning for structural tags, AFAIK

>Is there something wrong with adding an index to a table programattically and keeping the DBC in sync?