This conversion seems to represent a transformation of standard 3D Objects (.obj) to SpeedTree 3D tree graphics (.spt). It seems that you can do this conversion only with the original SpeedTree tool for creating animated trees. But perhaps some other programs may also support .spt files.
Convert 3D object graphics to SpeedTree 3D tree graphics.
We found one software available in the database for this conversion.
Microsoft Windows software - convert obj to spt on Windows
A tool for creating 3D animated plants and trees for games
Convert from obj file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.