How to transform 32 bit *.lib files to 64 bit *.a files

  32bit-64bit, c++, c++builder

I am currently porting a 32 bit c++ program (with C++ Builder) to 64 bit Code. The application is linking a 3rd party dll and the 32-bit compiler project contains the associated lib file, as usual.
In 64 bit mode, these ".lib" files need to be transformed into archives with extension ".a". I know there is a conversion tool for that (a simple executable) I have used once successfully a while ago. But I cannot remember the name of that tool. Can someone help me here?

Source: Windows Questions C++