Is this what you want? The following Syntax should be equal to what you are describing here:
edit: there is a very useful 2000 pages pdf syntax reference file available from ibm. it's great if you already know the name of a command and just need some details on it or how it works. link: IBM_SPSS_Statistics_Command_Syntax_Reference.pdfCode:
DO IF your_variable<=0. COMPUTE new_variable=$sysmis. ELSE. COMPUTE new_variable=1. END IF.