FxPansion VST/DX adapter (aka: wrapper) will do this. When you run the adapter it ask you to point to all the directories that contain VST/i's then it catalogs them as DX/i's with Windose. When you open Sonar open synthrack you will see a VSTi section, Fruity stereo and Fruity Multi out will be there.
When you click on the orange Fruity fruit, Fruity opens up as if it was stand alone. Set up your sequencing and synths and lay it out your song in the Playlist editor.
When you want to get back to Sonar just hit the standard "X" close button. Don't worry you won't loose anything (though I always save a Fruity version just in case). When you hit play in Sonar, Fruity will play in song mode also. In sync.
If you want to control a Fruity Channel in Sonar just insert a midi track in Sonar then select a midi channel number for that track. If you want to controll the 3rd "sound" channel down in the list in Fruity then choose Channel 3 in Sonar. You can also control levels/pan and other Fruity stuff from Sonar (but I don't know how to do that yet).
In Fruity you can assign different channels to seperate FX channels (16 max) and then assign each of those to a seperate OUT. You can then have those 16 outs assigned to seperate Sonar "in" channels so you can mix them in Sonar.
Personally I build in Fruity till the computer max's out then I batch render out of
the 16 FX channels and open up the 16 or so wav files in Sonar and continue from there.
Sonar/Fruity is a killer combo. Fruity has some silly signal routing logic such as using the FX channels (inserts if you will) as OUTs and the fact that the Channel volume is pre fx. But I believe they are working this out. The new Fruity 4 will be called Fruity Studio and will be more robust.
Good luck
jack