Considering .stw files are templates for text documents (from OpenOffice) and .xls are spreadsheets (from Excel), it does not seem likely to convert one to another or vice versa. You would need a spreadsheet template from OpenOffice instead document one.
Found 2 different conversion types using the same filename extensions.
Conversion stw to xls type 1:
Convert from OpenOffice.org text document template to Microsoft Excel 97 to 2003 workbook.
Apple macOS / Mac OS X software - convert stw to xls on OS X
A full-featured set of office applications for Mac OS X
File type (from-> to)
Convert from stw file format
OpenOffice.org text document template
Microsoft Excel 97 to 2003 workbook
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 stw to xls. 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.
Conversion stw to xls type 2:
Convert from Statistica workbook to Microsoft Excel 97 to 2003 workbook.
Microsoft Windows software - convert stw to xls on Windows
A suite of analytics software products