multiple parties to sign a transaction before being broadcast (or after, but either way be associated with) Lets say someone was writing a transaction solely to denote the existence of something, or a timestamp, or writing something in OP_RETURN

Could that transaction be signed by other addresses, kind of as a "I agree with this statement" way, or a way that future analysis of that transaction showed that particular parties agreed with the transaction

(I'm imagining something that does not require the sender to have a multisignature address created with other parties. Or some way of having multiple outputs from different parties going to one input)

For example, Tom requested the location of Atlantis, many parties are watching these requests on the blockchain and can collaborate on the answer. Eventually Tom received a transaction containing the location in the OP_RETURN and it came from NOAA. Tom doesn't trust NOAA's use of GPS and much prefers GLONASS, so this transaction was also 'signed' or endorsed in some way by the Hydrometeorological Center of Russia. "Wow both agencies agree on this, I'm convinced now"
You could do something along those lines:


Create a transaction to a multisig address with private keys that the different parties control.
The parties sign a transaction having for input that multisig (which shows they agreed and signed) and for output the OP_RETURN.