I have a theoretical way but i've yet to test it.
Basically it revolves around the XML export
1. Export the On-Access Default Process policy and the on-demand task to XML
2. Manually edit the XML and copy and paste the exclusion section from the policy to the task
3. Import the task
The exclusion section of each exported XML is that same format so should be transferable.
The whole idea is to NOT do what you've asked. If you exclude things from the On Access Scanner then you should definitley be scanning it with the On Demand Scanner. Otherwise you've created a permanent hole in your systems.