1. How can I get my RISA model into STAAD.Pro ?

There is no way to directly get a RISA model into STAAD.Pro. You may check whether RISA can export in the following formats, both of which can be imported by STAAD.Pro.

1.  CIS/2 
2.  3D DXF

Out of these, the CIS/2 is the best option as it lets you transfer not only the wireframe but other member information like Member properties, Material properties, Member orientation, Member end conditions like Releases, Support conditions, Loading information

3D DXF transfer will let you transfer the wireframe only.

These import options in STAAD.Pro can be accessed from within the top menu File > Import

P.S. Although this wiki is about RISA, but it applies to other analysis software as well.