In theory you could create a RAID 0 array between the built in storage for /opt and the microSD card. To do so you would have to compile or use already compiled raid drivers as a module or built into the kernel. I am not sure if the default kernel supports RAID but I assume it is removed to reduce size. You would need to format the RAID partition destroying any data in /opt or on the SD card. I feel this is a bad idea. If you removed the SD card /opt would be unusable. Also the SD card and built in /opt storage are likely to read and write at different speeds.

A better solution might be to link the SD card to a folder under /opt for example make it /opt/sd and store some files in there.

My intention is to remove the /opt partition and add that space to the root partition. Then use a partition on a never to be removed SD card as /opt.

It will much easier to figure this out once we have phones to try it.