Unfortunately you will not be able to set up SR Approvals for decommissioning as they can only be configured to have change approvals. The reason for this is the tool does not create an SR when a decom action is performed (same for start/stop and extend as well) therefore it cannot place an approval around it.
Many thanks for your answer!
Do you see any other solution e.g. via Change Management? Or is customization the only way?
You would need to use the Change Management integration in order to use approvals in this case. There are only 2 types of actions in CLM 2.x that spawn Service Requests: Submit and Post-Deploy Actions. For the rest of the actions (including decommission) you will need to use the CMF:ChangeIntegrationEnablement form to configure. I just posted a doc with some more info that should help you get it set up: https://communities.bmc.com/communities/docs/DOC-19692