It seems there is more than one choices for x_t to 3dm conversion, as several 3D formats use 3dm extension for their file types. Anyways, this looks like a 3D format conversion and as such you can do it with some dedicated 3D/CAD conversion tools like PolyTrans or similar programs.
Home > Search converters: x_t to 3dm
Conversion of x_t file format to 3dm file format beta
Search for x_t to 3dm converter or software able to handle these file types.
Found 2 different conversion types using the same filename extensions.
Conversion x_t to 3dm type 1:
Convert from Parasolid model (text) to 3D NURBS CAD file.
Microsoft Windows software 
- convert x_t to 3dm on Windows
PolyTrans|CAD+DCC
A software suite used for 3D CAD/DCC + Animation/Model/NURBS/Skinning translation, optimization and viewing.
File extension | File type (from-> to) | Convert | Open | Save | Edit | Create | Import | Export | Extract |
|---|---|---|---|---|---|---|---|---|---|
Convert from x_t file | Parasolid model (text) | No | No | No | No | No | Yes | No | No |
to 3dm file | 3D NURBS CAD file | No | No | No | No | No | Yes | Yes | No |
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned x_t to 3dm. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
On-line services software 
- convert x_t to 3dm on web
Babel3D
Babel3D is used to convert 3D files to 3D PDF, 3DS, COLLADA (DAE), OBJ, JT & STL. A translate only plan can be purchased for 14$ per month.
Conversion x_t to 3dm type 2:
Convert from Parasolid model (text) to QuickDraw 3D metafile.
Microsoft Windows software - convert x_t to 3dm on Windows
PolyTrans|CAD+DCC
A software suite used for 3D CAD/DCC + Animation/Model/NURBS/Skinning translation, optimization and viewing.
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