Hi Nasos Pap,
I believe the product that will help you is called the "BIOS Multi Core SDK" (MCSDK).
Please see this link for a release that supports the 6657 as well as some more info.
As far as migrating from a DSP/BIOS app to a SYS/BIOS app, yes, the legacy migration guide that you found is your best bet. I would recommend going through that and using that document to help guide you in migrating your existing BIOS 5.x app to work with BIOS 6.x. Please note that DSP/BIOS legacy support is being phased out in the latest versions of SYS/BIOS, but you should be OK with the SYS/BIOS version that was shipped with the BIOS MCSDK I linked above.
I also saw this related thread regarding porting of CSL ... not sure if you will need to do that but thought I'd list it since I came across it.
Steve