I assume that you mean a native code dll rather than a .NET dll. You can't directly call into native code from Basic4ppc, you would need to write a helper library using a .NET language, most likely C# or VB.NET, that called the external dll for you.
If the dll is actually a .NET assembly (like the Basic4ppc libraries) then it might be possible to call into that depending upon how the code in the library is organised and whether Basic4ppc supports the types of the parameters and return of the function you wanted to access.