You may have luck by providing a list. However, I see a deeper problem. Serializable is a marker interface: it has no methods, it only declares the type serializable. However, you cannot just declare that an object is serializable to make it so; all its components must be serializable as well. This includes the invocation handler that ABCL creates under the cover, as well as all the Lisp objects that you use for the implementation, particularly functions and closures. And, bad news – those aren't serializable. So, if Serializable is a requirement because those instances will effectively be serialized – e.g., to persist them to a file or to send them over the network – you're out of luck.
Ages ago I had started a branch to make most Lisp objects serializable, but I don't remember how far I got. I don't think it was ever mature enough to be merged, but many years have passed.