analyzing your tables should really make hardly any difference to the size of the tables in the system tablespace (some rows may grow or get chained in the worst case). Providing your system tablespace isnt 99% full you should be OK.
It's about 97% full. So here's my second question: to add space to the SYSTEM tablespace, should I extend the existing datafile or create a second datafile? Keep best practices in mind here, of course. BTW, ample server disk space is available for growth.
Do autoextend on and specify a reasonable maxsize using 'alter database datafile'. In this case I would avoid a second datafile as it makes fragmentation problems worse and it isnt going to grow much anyway.