Can I create a builder for an existing class that I don't control?

[xenoterracide]

So we've got this code generated from openapi-generators. I'd like to create a builder with defaults for it. I have no control over these classes right now though. Am I able to create a Builder for this? If not could we get this ability somehow?

[elucash]

we have something similar for importing abstract value types (as interfaces or abstract classes) from other packages, for builders alone, for those external classes which have constructors, we have @Builder.Include(<classes>) (from org.immutables:builder maven module) placed on a package in which builders will be generated. But I don't know how to handle / communicate what has default values/what default values are