Usually only with a wrapper or interface DLL. In general there are three types of DLLs.
"Normal" dlls that are just native code packages of precompiled functions with a list of entry points. Most of the Windows API dlls are of this form. These can be called by a .NET program using what is termed P/Invoke (Platform Invoke) which is similar to "Declare" in previous versions of Visual (and earlier) Basic. These dlls cannot be called directly from B4PPC but can by means of an intermediate library - like my Flite device speech library.
COM (Component Object Model) or ActiveX dlls are more complex beasts which support some object oriented programming features such as memory managment and reference counting. They can only be called from a .NET app via what is termed an "interop" assembly so they too need an intermediate library. My desktop speech library calls the "Microsoft Speech Object Library" which is such a COM dll.
.NET dlls or assemblies are precompiled .NET code like all the B4PPC libraries. In theory some might be usable directly by B4PPC but it will depend upon how they are organised. I have to admit I don't know exactly what the requirements are as I have only ever used .NET dlls that I have written myself.
P.S. My ControlsExDesktop library has a DateTimePicker that looks like the B4PPc calendar. You can add that at runtime. It is only for the desktop though as the library name implies.