gst-convert can be used. It also supports rewrite rules to fix dialect incompatibilities automatically. You can write a script called, for example, gst-squeak like this one:

#! /bin/sh
#
# usage: gst-squeak SOURCE DEST
#
# Converts Squeak code to GNU Smalltalk

exec gst-convert -f squeak \
  -r'Float->FloatD' \
  -r'BlockContext->BlockClosure' \
  -r'MessageSend->DirectedMessage' \
  -r'DateAndTime->DateTime' \
  -r'TimeStamp current->DateTime now' \
  -r'TimeStamp->DateTime' \
  -r'ProtoObject->nil' \
  -r'UnhandledError->SystemExceptions.UnhandledException' \
  -r'Date current->Date today' \
  -r'(``@object and: ``@arg1 and: ``@arg2 )->
     ((``@object and: ``@arg1) and: ``@arg2)' \
  -r'(``@object ifNil: ``@arg ifNotNil: [ | `@t2 | `@.s2 ] )-> 
     (``@object ifNil: ``@arg ifNotNil: [ :foo || `@t2 | `@.s2 ])' \
  -r'(``@object ifNotNil: [ | `@t2 | `@.s2 ] ifNil: ``@arg )->
     (``@object ifNotNil: [ :foo || `@t2 | `@.s2 ] ifNil: ``@arg)' \
  -r'(``@object ifNotNil: [ | `@t2 | `@.s2 ] )->
     (``@object ifNotNil: [ :foo || `@t2 | `@.s2 ])' \
  -r'(``@object ifNil: ``@arg1 ifNotNilDo: ``@arg2 )->
     (``@object ifNil: ``@arg1 ifNotNil: ``@arg2)' \
  -r'(``@object ifNotNilDo: ``@arg2 ifNil: ``@arg1 )->
     (``@object ifNotNil: ``@arg2 ifNil: ``@arg1)' \
  -r'(``@object ifNotNilDo: ``@arg2 )->
     (``@object ifNotNil: ``@arg2)' \
  -r'(``@object doIfNotNil: ``@arg2 )->
     (``@object ifNotNil: ``@arg2)' \
  -r'(``@object newFrom: ``@arg2 )->
     (``@object from: ``@arg2)' \
  -r'(Dictionary withAll: ``@arg2 )->
     (Dictionary from: ``@arg2)' \
  -r'(``@object evaluateWithArguments: ``@arg2 )->
     (``@object valueWithArguments: ``@arg2)' \
  -r'(``@object beginsWith: ``@arg2 )->
     (``@object startsWith: ``@arg2)' \
  -r'(``@object allSubInstancesDo: ``@arg2 )->
     (``@object allSubinstancesDo: ``@arg2)' \
  -r'(``@object directoryNamed: ``@arg2 )->
     (``@object / ``@arg2)' \
  -r'(``@object fileExists: ``@arg2 )->
     (``@object includes: ``@arg2)' \
  -r'(``@object readOnlyFileNamed: ``@arg2 )->
     (``@object / ``@arg2) readStream' \
  -r'(``@object forceNewFileNamed: ``@arg2 )->
     (``@object / ``@arg2) writeStream' \
  -r'(``@object caseInsensitiveLessOrEqual: ``@arg2 )->
     (``@object <= ``@arg2)' \
  -r'(``@object isZero)->
     (``@object = 0)' \
  -r'(``@object recursiveDelete)->
     (``@object all remove)' \
  -r'(``@object containingDirectory)->
     (``@object parent)' \
  -r'(FileDirectory default)->
     (Directory working)' \
  -r'(``@arg2 assureExistence)->
     (``@arg2 createDirectories)' \
  -r'(FileDirectory on: ``@arg2 )->
     (``@arg2 asFile)' \
  "$@"