an over-size quick link (e.g. a Suncor SS one) with a rating greater than that of the chains it's joining is a possibility. But, of course, if you have a windless that will be a problem. I agree in general it's better to buy a new chain BUT suppose your main chain is good but you want to simply increase the length of the rode to allow for greater scope in an anchorage that's relatively deep? In other words, it can be a short-term solution - even if it's not a good day-to-day set-up. Everyone should have some extra chain (and spare line rodes) stored aboard anyway.

Jerry