HI Mufeed,
Looks like you are confusing standard User Decisions with other types of tasks.
FIrstly you do not usually have to change your existing workflow at all to work with Fiori My Inbox
Workflow user decisions are BOR based & use outcomes as the answers.
These outcomes are automatically mapped to buttons in the Fiori My Inbox app
However for any other type of workflow task, including ABAP Class tasks Fiori My Inbox gives 3 options for actioning the task
1. For simple actions such as approve/reject configure a decision key for each button you want displayed for the task in the My Inbox configuration & implement them in the BADI i.e. Use the BADI to call a background method to perform the data updates that match the button. Eg an approve button would heed code in the BADI to update the object to set it to approved status
2. For more complex tasks, Use the standard Open Task button & intent navigation to launch to another app where the task is performed
3. For tasks where you want to tailor the UI further, use the inbox extensibility.
None of the above options require changes to your workflow
Jocelyn