incorporate 2 channel amp into multichannel system

Gentlemen:

What you have described is the procedure for emulating a HT Bypass when that function is not provided. It is an input on the 2 channel preamp with an accurately fixed gain into which one feeds the L/R line level outputs from the HT or MCH processor. Then, setup for each is handled independently but the L/R signals are always routed through the stereo preamp.

I use a Parasound JC-2 BP for this and it is fed with the L/R outputs of my Meridian 861 as well as directly with analog stereo sources. Related stories here:
http://www.stereophile.com/content/music-round-47-page-3
http://www.stereophile.com/content/music-round-34-page-2

So for example if I connect a cd player to the creek I can determine with a spl meter where 0db is and then mark it as my ht volume. Is that correct?

No. First you need to get a multi channel setup disc. Next you play it in multi channel player connected to your SSP. Then you calibrate your channels individually, using the channel trim feature on your SSP. You could do this by setting your center and mains to 0db offset level on the SSP, and then find the volume setting on the creek that gives you the same SPL reading for the mains and the center. This may or may not be exact unity gain on the creek, but that does not really matter, as long as your 6 channels are calibrated correctly relative to each other.

That's one way. The simpler is to connect the L/R outputs of the SSP to the Creek, set the latter at a mid-level of the VC rotation and mark it so you can repeat that setting accurately. Then you run the SSP's setup/calibration for all channels via the SSP and the Creek. Done.

The only issue with that approach is that "mid level" may be too much attenuation. If 12 O'clock is say -20db, then you would have to crank up the mains by 20db on the SSP channel trim settings, which may be out of range. If you take your approach setting the creek at 3 o'clock is probably closer to the unity gain mark.