I tested Psion Sheet import back in the 1.0.x era with sheets written by the Psion Netbook, but it didn't work for me. Inquiries to Psion about documentation of the file format went unanswered.
The documentation about the psion sheet file format you may be find at http://www.frodol.dds.nl/psiconv/html/Index.html.
Regards Ulf