Only thing i can think of is do a #finditem (spellname) to find what the item id of every spell you want on teh merchant, then go through and make a custom merchantid and import all your items onto a 'custom' merchant, thereby being able to customize the merchant to sell all the spells instead of just teh some...
If you can find a merchantid table somewhere (i haven't, but then again, i haven't looked heh) you could find out which merchantid numbers have the current spell list, then just edit them to include the spells that are missing... but personally it seems easier to me to build a custom merchant, even with all that hand editing... /shrug i've put up over a dozen merchants each selling 30+ items by type (all blacksmithingmade weapons on these 2 merchants, all blacksmithing chain on this merchant etc etc) in the last 3 days and it's not so bad.... although the more i look at the sheer quantity of player crafted gear (which i'm trying to put all of onto merchants) the more it makes me wanna cry
Anyways, hope that helps, if you need more info on how to actually make custom merchants pm me, or just do a search, i'm sure theres a half dozen threads on it (too lazy to go find one right now tho