There's no option for adding a "monthly" post paid unlimited plan onto a prepaid account,
however prepaid does have lot of solutions for unlimited calling within Canda, or getting additional minutes, called boosters as add-ons for prepaid.
Unlimited Messaging (Text and Picture)2
Unlimited Local Calling (Incoming and Outgoing)7
In addition, you can choose from several prepaid plans, Skye, such as the $25 base plan will give you unlimited local (!) 7pm/weekend calling, the $35 will give you 5pm/unlimited incoming and the $50 base plan unlimited local calling :)
Of course all of them feature unlimited texing!