If I join a fraternity that doesn’t party with a certain sorority that I have friends in, will I lose touch with those friends?
No! They’re your friends - nobody should be dictating who you are friends with!
I can’t specifically speak for your school, but all schools I have visited/ know people at it should be no problem to have friends in different sororities/frats. If you want to be friends with them you will be able to do that. Contrary to the (usually false) stereotype of the “overly cliquey” nature of greek organizations, people can make friendships outside of your particular frat.
I’ve made a lot of friends in different frats and sororities. Just because they aren’t in your house doesn’t mean you can’t still be friends with them. In fact most chapters encourage you to have non Greek friends as well. It would be rather boring if you only knew people in your house.