I have a new pc with 1 hdd and W7 installed (preloaded) and I'm planning to add another disk (SSD) where I have BM and eCS already installed. Since I doubt that it is just to plug in the SSD disk and add W7 to BM I would appreciate some advice.
It's exactly that easy for once

I have the exact setup here, so it should be easy to do.
Win 7 has no problems booting after moving disk 1 to second position here.
1) Don't touch the disk with Win 7 on it.
2) Install your SSD disk in first position.
3) If the IBM boot manager does not work, just install Air-Boot as the boot manager.
4) Add the Win 7 boot partition to Air-Boot
And that's it !!!
Here is some link from me messing around:
Dual & Triple Boot
http://www.os2notes.com/os2dualboot.htmlhttp://www.os2notes.com/os2tripleboot.html*** Note: I used custom install to install Win 7, so I only have one partition on my disk for Win 7. When its pre-loaded Win 7 usually has two parition, so I'm not sure how that will work. However, it should work the same way - just point Air-Boot to the small boot-partition of Win 7.
Cheers,
Greggory