Could you explain what you mean by linking to existing work packages? Are these items in Integrity?
Assuming the task you are creating is an Integrity Item and you want to link to another Integrity Item:
On the CLI, after creating the task, you can edit it and change particular fields.
To link to another Integrity Item, you can add a relationship to that Item. For example: "im editissue --addRelationship="<Relationship Field Name>"=<Related ID> <Item ID>", where <Relationship Field Name> is the name of the relationship field, <Related ID> is the ID of the item to relate, and <Item ID> is the ID for the task.
For more information on the im editissue command, see the CLI Reference in the Online Help: https://support.ptc.com/cs/help/integrity_hc/integrity111_hc/en/#page/IntegrityHelp%2Fim_editissue.html%23
