This makes no sense; extending C code with Checked-C would still be easier as with Checked-C you wouldn't do any translation at all; it's a superset of C, in the same way that TypeScript is to JavaScript.
Second, you can't just add checked C to a codebase, because you have to actually interop with the unchecked C code, which uses different pointer types.
My argument is that it may be easier to do an automatic migration to unsafe Rust, and then incrementally convert to safe Rust, than it would be to leave the codebase as C and try to incrementally convert it to checked C, because the "incremental conversion to checked C" is extremely difficult.