Addendums have to be enabled for EZSign. Most are. Some are not. If you find a form or addendum not enabled for EZSign, you would need to create a ticket with EZLandlord and ask them do enable it. It might take them quite some time to do it, if ever. Alternately, use chat and ask the CS rep to create a custom form for you with the EZSign enabled. I believe they can do that.
how do I edit a lease already emailed to the tenant but not yet signed
You have to cancel the EZSign, and then re-edit the lease, and re-save the lease with EZSign.
Click HOME at the top left of the page. Click CANCEL EZSIGN to the right of your lease.