RISA has a number of export options which can work.
Export - DXF file: STAAD should be able to read/import this type of file. Only problem is that this bring over only geometry... not member sizes, not loading.
Export - SDNF file: I don't know if STAAD can read this type of file or not. But, I'm guessing that they can. This should bring over geometry and member size information. But, does not bring over loading.
CIS/2 Translator: This is a free utility program available on the RISA website. It should bring over everything. If STAAD can import it, of course. At some point, I think STAAD had a CIS/2 translator. I want to say that it was a separate utility you had to buy from them. But, I don't know for sure.