There is no equivalent functionality in Basic4ppc (nor in the .Net Compact Framework).
If you can settle with checking the files about once every minute or more then you can use FileSearch to add the files to a Hashtable. After the first time, every file that FileSearch finds and is not found in the Hashtable you can consider as a new file.
.NET Assemblies whose classes' constructors, methods and properties use strings or primitive value types. For events to work the library must implement the Basic4ppc event convention.
Can I use any other library developed for VB in Basic4PPC?
If you mean already written libraries then possibly but fairly unlikely. If you mean ones specially written in VB then yes. I've had a look at the FileSystemWatcher code and, unlike some other bits of OpenNETCF, it looks like a fairly easy job to implement as a Basic4ppc library if there was enough demand to warrant the effort.