yes, eCart can be used with a subscription site.
you would need to use Security assist to create the registration portion.
basically you would be using security assist User Level Access (there is a tutorial for this in the archived documentation section of the security assist support page).
the user would register for an account on the site, on initial registration they would be given a restricted user level and they would have the option to purchase access to the paid content on your site.
using eCart, they add the paid member ship item to the cart, then checkout. once the transaction completes successfully, the user level is updated to allow access to the paid content.
there are tutorial on the eCart page for adding registration / logon to the checkout pages
