Absolutely. Almost all of the true CCaaS options have ‘overlay’ capabilities. Which means they can bolt onto or interface well with an existing PBX. Some of the CCaaS players even have preferred vendors and built in connections with many of the key PBX providers.
Best option, of course, is to purchase a CCaaS solution with a built-in PBX. But when that is not an option, there are several great choices for add on CCaaS solutions.