Go to a local jeweler and have him or her solder on some sterling silver end caps/tabs - these work with flat style chains. A sterling lobster clasp can then be added through one of the end tabs, and you could probably even add an extension chain so that you can adjust the length of the necklace (not needed, just a 'nice to have' if you wanted to be able to wear the necklace at different lengths and with more styles of tops/dresses).
Here's a couple of photos of silver end caps/tabs (shown below in pic #1 and pic #2).
Here's a whole screen full of them:
http://www.riogrande.com/Categ.....r-Bars/494
I would take the chain/necklace to a jeweler and ask them about changing out the clasp. I wouldn't recommend doing this yourself, because if the chain is flat, then you'll want an end tab/end cap soldered onto the chain - it's not like you can just use some pliers and loop a lobster clasp onto a flat chain, if I'm reading your post correctly about the style of necklace. Sounds like something like photo #1 might work, but your jeweller will need to order the correct size for you depending on the width of your chain, and you will want it professionally soldered. As you can see, there are little round rings at the end of the end caps through which a lobster clasp can be added. This is going to be a lot easier to put on (and stay on!) than fussing with a box clasp.
Good luck!
---> editing to add/ask: are there any links at all at the end of your necklace? If so, you won't need an end tab/cap - a spring ring or lobster clasp can be added through if there are chain-like links at the end of your necklace. If it's a flat type of chain, then you'll want one of the end tab style findings attached to make way for a lobster or other clasp.
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