It seems that old Windows Mobile skins and themes (.tsk) are compressed ZIP archives that contain the graphics that changes the visuals. There is no need to convert tsk to zip, just rename the files to zip and extract them with any tools available should you need it.
Convert Windows Mobile skin or theme to WinZIP Compressed ZIP archive.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert tsk to zip on Windows
A compression utility for Windows, ZipZag offers very large support of compression formats. ZipZag opens standard Windows/DOS archives, Linux/Unix/Mac archives, skins, themes, collections as well as games and applications data files.
Convert from tsk file
to zip file
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.