Theres a bunch of things you have to keep in mind when adding mods to an existing modpack this way, this is not an exhaustive list but think about:
1) Target verson of Minecraft for the mod packs youre lookiing at... If they're both running a different version of Minecraft then theres a very good chance that the version of the mod you want to steal from one and place in the other wont work...
2) Dependancies of the mod you want to "steal", in this case you're looking at ICBM by Calclavia, right on his website he mentiones: "This mod is dependent on Universal Electricity Core, and Calclavia Core" It would be worth verifying that the version you're looking at is indeed dependent on thses, determine if they're in the target modpack, if not you'll need to steal them too.
3) ID conflicts. If the mod youre stealing defines a block ID = 12345, and you've already got a mod that defines that, bad things will happen, the least bad of which is you'll never see one of those blocks in your game.
4) If you mess with mods that are defined in a modpack managed by the technic launcher (like you want to redefine "Tekkit" in your launcher to have a new mod) its really safest if you create your own pack, "steal" everythign from tekkit including the configs which take the longest to deconflict and set up nicely and add the mods you like... Define the thing as a different modpack alltogether. This will avoid issues with updates, or anything that could crop up because of the discrepancy (not the least of which is that you'll have issues running or joining SMP Tekkit servers if you've got a different set of mods...
Once you make sure all thats taken care of, its a matter of dropping the .zip or .rar (typically) into the proper directory and the mod loader will pick it up and install it... #1, #2 and #3 can all cause errors starting up a server before you ever see the actual game...
Edit: I'm at work now or else I'd tell you what to copy and change to get that in there... but based on the (crappy, sorry) info on the website, Tekkit has none of the pre-reqs, and the version of MC its targeted at is 1.5.1 (which I didnt think was correct, you can see this easily in the launcher) and the link to the "official volts wiki" is broken...
Edit again: The only version mentioned on the Tekkit modpack page is on an old video they still have up saying 1.5.1, however all the other info I find on other sites says its 1.6.4 (which I would expect) The Technic Voltz page has an ancient version of ICBM, and doesnt even list the dependancies on the site...
