WordStar for DOS
Microsoft RTF files can be created using Star Exchange, which was supplied with later versions of wordStar for DOS, and with WordStar 2000 for DOS. If you don't have this program you can try WS-CON.
Verity's KeyView Pro doesn't have any WordStar specific file import filters, but can read WordStar for DOS 3.x and 4.0 files as these are relatively simple text formats. These can then be saved in Rich text Format (RTF). KeyView Pro offers a batch conversion facility. A demo of the program is available from www.verity.com/keyview.
SoftWare Bridge was customised and supplied with both WordStar for DOS versions 6 and 7. The program is no-longer sold, but copies may occasionally appear on on-line auctions such as eBay. If you look for this, try to get version 5 as this included Rich Text Format (RTF) conversion - alternatively, look for copies of WordStar for DOS version 7.0d as these included the RTF supporting version of SoftWare Bridge.
Conversions Plus is a specialist file conversion program sold by DataViz. It only supports the following WordStar for DOS versions 5.5, 6.0, 7.0; although earlier versions could be opened as text. The program costs around $70 for a single user licence (Mar 2001) and is available from DataViz at http://www.dataviz.com