How can I reconfigure my breakers to make room for a sub-panel feeder?
There are 100a 32slot main panels available from GE. This would solve your lack of slots, but since you want to add a 60 amp service it may be wise to upgrade your service to 150a with a higher slot count. Many electricians will suggest going as large as possible for future loads.

If you do change from an electric water heater to gas you could use that slot but many water heaters no longer have pilot lights and they require an outlet for the igniter, so keep that in mind. The wires in use now may be able to be changed to 120v depending on the type (some only have 2 hots and a ground). If legal in Canada you could reidentify one of the hots as a neutral at both ends so you won't have to pull new wires.

I have heard that the Canadian federal Pacific panels don't have the issues experienced in the US but if it is a stab lock style of breaker I would want an updated panel because I have found several that have failed for breakers not tripping and arcing at the buss. I save used breakers for possible use on other panels but after my 3rd stab lock panel failure I started testing breakers and found quite a few failures. I no longer save stab locks. I toss them and the only work I will do on a stab lock panel is to replace it because of liability.