RE: Assign a new user/approver through code x++ to workflow after submitting

What is the actual problem or requirement here? You would be deciding workflow approvers on the fly? You can think of delegation feature of standard AX. Delegate method of SysWorkFlowWorKItem class

SysWorkFlowWorkItem::delegate(workflowWorkItemTable, WorkflowWorkItemDelegationParameters.User,

                                                                  WorkflowWorkItemDelegationParameters.DelegateTo, 'Delegation');