If you don't need to worry about dealing properly with (hard) tabs, then you can easily write a short command to do the reformatting.
Set input to "Entire Document", output to "Replace Document", and put the code
#!/usr/bin/perl -p print "$1\n" while s/(\S{$ENV{TM_COLUMNS}})//; print "$1\n" while s/(.{0,$ENV{TM_COLUMNS}})\s//;
into the Command(s) box. This will treat a tab as though it took up a single column, so if you have a lot of tab indents then it will leave excessively long lines. Writing something that deals properly with tabs would be a bit harder, but very possible.
(I bet somebody has already done it too!)
Robin