a P2SH is exactly that: PAY TO SCRIPT HASH.

In other words, the script is set in stone the second you fund it.

If you don't fund it... there's nothing to sign.



So in other words, no, you can not do the situation you described.

The flow works like this:

I make a P2SH multisig that is m of n.

I gather the pubkeys of the other n - 1 participants, and I use their pubkeys along with mine to generate the script, and hash it.

I then get a P2SH address. (which is a hash, so unchangeable once finalized)

Then I send bitcoins to that address. At this point the n, the m, and each individual pubkey are set in stone. I can not change the pubkeys out with other ones, I can not change the number of signs required... because the hash already has bitcoins sent to it...

the script that you are hashing has m, n, each pubkey, and the checkmultisig op code in it.