There is no such tools for converting Swift code to Objective-C. There are some reasons. One is because Objective-C is a semantic subset of Swift, i. e. Objective-C doesn’t have any equivalent of some fundamental features of Swift such as Generics, Algebraic data types, Pattern matching and other. Theoretically it is possible to make, but generated Objective-C code should be boilerplate and inefficient.
It is recommended to migrate your codebase to Swift to avoid technical debt there are many tools which can be used to convert Objective-C to Swift.