No – you can’t add items to the agenda once the notice of meeting has been sent out.
The Act provides a mechanism for putting motions on the agenda for the AGM. If you want to have motions put on the agenda, send them to the strata manager and cc the secretary of the Executive Committee, and tell them you are doing this in accordance with the Act.
You have some idea when the meeting will be held, so make sure you send them about six weeks prior.
Re the EC being uncommunicative, they are required to give 72 hours notice of any EC meetings and you are entitled to attend those meetings. If they are not giving notice of the EC meetings you should ask why. If they are not providing minutes of the EC meetings you can also ask for those.