I found a TSX plugin that exports an X model out of Truespace to 3D Exploration for conversion to BZII XSI. This plugin is available for download with usage instructions on my modeling forum. The exporter exports the model without renaming any model parts, so that when the model is converted to XSI with 3DEX it will directly create a usable BZII model without any further work. The plugin was originally written for Truespace 5, but I'm using it with Truespace 6. Anyone using Truespace 4 is also in luck as there are 2 TSX versions included in the zip folder along with a readme.txt file. Click on this link for more information and to download the zip file with the TrueX TSX plugin. Unzip the files to your Truespace TSX folder and click on the Electrical Plug icon to install it.
Update: Scratch this exporter off as a no go for BZII usage. I tried it out and the X format is too new for 3DEX to convert properly to XSI with 3DEX, it will convert the model X to XSI but no textures show up on the model in the Map Editor. The models themselves work okay in terms of weapons and such but only the base material colors show up in the Editor.
FYI: The X version is 3.3 so you may want to keep it for use with newer video games that support the X format.