@liav To correct @cocreature slightly, you have to ask hub to create a multiparty token for you, which you can do using the cli damlhub-cli - npm, please see
damlhub-cli ledger multipartyToken
With that token you can make submitMulti requests as those are natural requests via the LedgerAPI. The only wrinkle is that all of the parties that you aggregate into a single token must be controlled by the party that created the ledger; which I think is true in your case.