StarOffice 5.2 can convert WordStar documents, however, I do not have any WordStar documents to test it on.
I know this as I downloaded and tried it a long time ago when Sun Microsystems offered it for free.
Assuming that you have the free version of StarOffice 5.2 for Windows somewhere, run
so-5_2-ga-bin-windows-en.exe
Or if you had downloaded each individual part, run:
so-5_2-ga-bin-windows-en-000.exe
For the parts installer to work, you will need these files:
so-5_2-ga-bin-windows-en-001.bin
so-5_2-ga-bin-windows-en-002.bin
so-5_2-ga-bin-windows-en-003.bin
so-5_2-ga-bin-windows-en-004.bin
so-5_2-ga-bin-windows-en-005.bin
so-5_2-ga-bin-windows-en-006.bin
so-5_2-ga-bin-windows-en-007.bin
In addition, you might also want/need:
StarOffice 5.2 Player
sop-5_2-ga-bin-windows-en.exe
Database (Adabas D)
soa-5_2-ga-bin-windows-en.exe
You can find more information about StarOffice here:
http://www.sun.com/software/star/staroffice/