Do you want to make something transparent in-game? If yes, then it is an exporter setting.
If you are using version 0.97b3 of the DTS exporter, then you can set up transparency the following way:
1. Under the Shape tab, find the Materials tab
2. Select the material you wish to make transparent
3. Click on "Translucent" and either "Additive" or "Subtractive"
4. Enjoy