Well, you can't literally "convert" the DLL, but there are a couple of approaches. One way is to write a Managed C++ wrapper that calls the DLL, and then call your Managed C++ routines from your native C++ code.
https://www.codeproject.com/Tips/695387/Calling-Csharp-NET-methods-from-unmanaged-C-Cplusp
The task is easier if the DLL is a COM server. Is it?
Tim Roberts, Driver MVP Providenza & Boekelheide, Inc.