incorporate 2 channel amp into multichannel system

Minkwelder is almost correct with one caveat. You need to trim channels levels on the receiver (using an SPL meter), based on the volume setting on the line input on the Creek. if a 12:00 setting represents say -20 db, you would have to offset by adding 20 db on the SSP which may be out of range. You will probably need to find something closer to unity gain on the Creek. However since you have no step attenuation with a volume level readout display, you need to set it at a high level (say 3:00) and possibly mark the correct volume level setting for HT mode with a piece of tape.

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.

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.