To answer that properly needs a bit more detail about the reason for the question. The short answer is that Basic4ppc libraries are .NET libraries (technically they are .NET Assemblies) so yes, but I suspect that is not what you meant
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If by libraries you mean the .NET Base Class Libraries, like System.Collections and System.Drawing, which underpin the .NET Framework then no, you can't access them directly but need a wrapper library written in a .NET language, preferably C#. The StringsEx library that you mentioned in another post is a wrapper round System.String to expose more String functionality to Basic4ppc.There are two reasons for preferring C# over the other popular alternative VB.NET. Firstly VB puts extra helper classes of its' own into a library which are visible as unnecessary objects in Basic4ppc Tools -> AddObject and secondly (more importantly) the C# source code can be compiled by Basic4ppc into the final application whereas VB libraries have to stay as separate individual entities.
If you mean libraries written by other people but not specifically intended for use with Basic4ppc the answer is a qualified maybe. It depends upon how the library has been structured by the author. You may be able to use some of the classes in a library in Basic4ppc but not others. You will almost certainly see loads of objects (classes) in AddObject that are of no use. Also events generated by such a library will not be raised in Basic4ppc as a library needs an event architecture specific to Basic4ppc for the events to occur within a Basic4ppc program.