Thomas Holder made a plugin for Inkscape to import/export .th2 files from/to Therion. Because internal format of Inkscape is SVG it is very useful for importing survey drawings exported from other drawing programs into Therion.
Plugin: http://www.thomas-holder.de/projects/inkscape-speleo/extensions/th2-latest.zip
How-to: http://therion.speleo.sk/wiki/doku.php?id=inkscapeplugins