You have a few options to warm-up a share list in your nation: 

• Get the endorsing organization to send the blast for you: rather than just sharing a list, your ally can email blast their supporters with your signup page. You can collect the opt-ins and treat them as your own supporters. This lets your ally respect his supporters while giving you the best chance for actual engagement from the new list. 
• Scrub the list using Accurate Append and do an opt-in campaign: often times, shared lists are years old, which means you have to scrub the list to remove outdated emails. Not doing so can damage your email reputation. Afterwards, do an opt-in campaign to ask the list to signup on your site for updates. Do not repeatedly blast those who do not opt-in since this increases the liklihood of spam reports. 

Getting a shared list isn't a replacement for building your own. An organic list takes time to build and a shared list shouldn't be considered a short-cut.