Update Workflow container in wrokflow step

Requirement:

Capture the reason for rejection in User decision step and send a SAP mail to the Initiator.

Steps in Workflow

1. user decision (Workflow container = ATTACH with 255 lines/ multiple)

2. Mail (assined in progam exit Tab = class/ Contains logic to collect all the data from Attchement of Workitem to an internal table)

          Now this internal table data I want to append in workflow container.

3. which I will use in mail.

I used SAP_WAPI_WRITE_CONTAINER  ---> Does not work

Return Code =  900

error 209

error message Fm cannot be called

can some one suggest how to go head with this.

Without editing Standard BOR.