The W200 will not Join an Existing SmartThings Thread Network and SmartThings cannot Join the Thread Network Broadcast by the W200 Hub
From what I can tell the Aqara app has no way to unify with SmartThings.
I’ve even tried joing the Aqara network from the Samsung hub but it tells me :
“Thread networks without a Thread 1.4 border router aren’t supported.”
You’ll need a Samsung Hub to join the SmartThings ecosystem.
OP is actually asking how to make the Thread border router (TBR) of the W200 hub and the TBR of the SmartThings hub share the same Thread network so they create a stronger mesh and a more reliable network.
While ideally the joining is automatic when the preferred Thread network is correctly stored in the phone, it does not always happen. That’s why Thread 1.4 introduced a way to manually share credentials with a one time code so a given TBR can join an existing network.