I am wondering if you can use MSN on a BlackBerry? The app is on the phone already but of course no server. 

probably no, but that would be cool

the only way you might be able to do that would be if you use it over wifi (probably possible),  and then use a transparent proxy on the router it's connected to which redirects the  MSN server DNS requests to the escargot ones.    Ironically i have a blackberry  9670  flip phone that has it installed as well. it would be fun to use. 
The only other way would be to write a  Web Messenger that would work with the  browser that the BB6  OS uses.  Then you connect through wifi.  
My current pet project though is recreating the  Blackberry 9670  phone  except with an ESP32-P4 inside instead of whatever  the original phone uses. writing a messenger app using LVGL would not be too complicated....