Hi Siva,
Are you using Process Controlled or Application Controlled workflow?
If it is application controlled please try implementing the BADI BBP_WFL_APPROV_BADI
If you are using Process controlled please try implementing BADI /SAPSRM/BD_WF_ADHOC_AGENT_F4. this gets called whenever you search for approver and / or substitute.
You can then define your own logic here.
Hope this helps.
Regards,
Karthik Babu