Can Isabelle's custom functions and data types be combined with the C-parser?

[riyuejiuzhao]

When verifying C code, some functions are difficult to verify, but I can ensure their correctness through other means. Therefore, I hope to directly implement the functionality of these functions in Isabelle and use them to verify other functions that use these functions. How should I proceed?
I tried to remove the function definition in the C language and define one myself, but the result shows: Duplicate constant declaration.
How should I solve this issue?
Thanks