I'd like to know if Sonar X3 Studio softwares are able to handle multitrack files created by CUBASE AI 6. It is because I have a Yamaha mixer that is equipped with USB digital connection to computers that interfaces with it through this software but its resources are really limited if compared with Sonar Suite.

No.

By other hand, is there Sonar driver for the Yamaha's mixer MG206C-USB? I am wanting so much to buy SONAR X3 Studio but I will generate my files using this mixer - I can play the guitar, bass and other instruments and I'm a singer. Is it viable to use Sonar with this mixer?

It appears to have an asio driver via usb. 
http://download.yamaha.com/search/detail/?language=en&site=usa.yamaha.com&asset_id=21044&category_id=16506&product_id=651562&1394389081
 
If its not locked to Cubase it should work in SONAR and allow you to record from the mixer via ASIO. You should check with Yamaha support.

I was going to reformulate the question when I received your response. In fact, I have a Audio driver for the USB mixer port not for the mixer itself. It is only pending to confirm if it will work with SONAR or if it is blocked for third part softwares. I will install it in Trial mode and test and if it works I buy the software. I would like to thank for your attention and I will give you a feedback, if it is ok or not, for future users support.

Whether it is locked or not seems a question for Yamaha. But I personally doubt it. As much as they might like to push Cubase, I'm sure they'd rather sell more mixers.

If it does not work, you might try saving the Cubase project as an OMF file and open that in Sonar. I have good experiences with that using Logic.

If the Mixer supports the mackie protocol you should be able to control the mixer aspect of the hardware using the Mackie control surface in SONAR via MIDI.

Both Cubase and Sonar support OMF and one can transfer projects back and forth using the OMF file format. 

It's the AI version of Cubase so no omf support.

When I was feeling that I was going have any hope to solve the problem, it seems dark again. Anyway, if I have to upgrade to an upper version of Cubase I would prefer since it seems to be cheaper than to exchange the mixer now . I will try these options and inform to everybody - softwares trial versions are exactly for this... to test and decide. Thanks for all. For sure, I will prefer keep this friendly support relationship and take advantage of the invaluable resources of SONAR.