The #Smalltalk compiler allows you to use classes from ordinary .NET programs. In fact, many of the standard ANSI Smalltalk classes provided by #Smalltalk are just wrappers around standard .NET classes. For example, the OrderedCollection class is a wrapper around the ArrayList provided by .NET.
#Smalltalk has a class library that is mostly compatible with the ANSI Smalltalk standard. Therefore, you should be able to run most ANSI compatible Smalltalk code. While the class library includes classes for the ANSI standard, there aren't many classes for other items. For example, currently, there aren't any classes for GUI development. It is planned to add such support, but so far it hasn't been done. What do you expect from a few months work?