This feature was removed November 2006
It has been superceded by plugins, because nobody seemed to care about this feature. Plugins are far less easy to write, especally since you need to know Java. If anyone is interested, ask [user:Imi Imi] politely to provide a plugin which calls an external Tool using the old XML-Syntax.
JOSM support calling to external tools. Currently, there is no GUI available to setup such tools. To enable this feature, create a file called "external_tools" in your josm preferences directory (~/.josm/ or C:\Documents and Settings\loginame\.josm\) that looks like:
<tools> <tool name="insert the name of the tool" exec="insert the complete command line to call the tool, including arguments" in="any of the words all,selection,screen seperated by ," out="either the word 'selection' or 'replace' or no attribute at all" flags="any of include_back_references,include_references,gpx" /> </tools>
The correct attributes needed to execute a tool can be obtained from the tool-Writer or by editing this page and help me documenting ;-)