Is there configuration to allow a posting consumer (OMM) to post to the ADH cache even if the RIC doesnt exist?
Yes this is possible - i.e. creating new records in an ADH Cache via OMM Posting.
Please refer to the ADH Install guide section OMM Posting and Inserts (exact location will depend on the version of ADH) for further details. There will be a subsection Configuration which details the parameters required (e.g. on the ADH v302 Install guide this is section 7.19.2)
You may also be interested in the next section of the guide Auto-Expiry of Posts... which allows to configure auto deletion of posted items after a configured time or cache size limit is hit.