Yes , You can move files from one disk to another withing OS .
However, you might need to reclaim the space to reflect it in the datastore.
It just moving files , if it's a Linux OS
mv (source location) (destination location)
obviously destination should be in a folder in "/dev/sdb" , as per your case.
The same way as you would if the guest OS was installed on a physical machine rather than it being in a VM.
(This isn't a VMware-related question)
Looks like I didn't describe it properly. We want to copy everything from 'dev/sda1' to '/dev/sdb' and also make sure that when VM comes up it boots up using '/dev/sdb'. Also, after little bit of investigation, I found it that '/dev/sdb' is just inserted and not mounted. Wondering if there is any easier way through ESXi to perform these tasks.