I already have 64 channels of MOTU Stage 16B, can I use CS18AI as a control surface with any AVB device?

No.

CS18 is not IEEE 1722.1 compliant in regards to AVB. It does not 'see' other AVB devices, nor can other AVB devices 'see it.'

CS18 can only control RM Series mixers.

MOTU would need to support the CS18 as a control surface for their hardware, and vice versa for CS18 needs to support MOTU's hardware. Neither of which will happen, as CS18 is a 'closed' AVB product.