I would say the major implication of upper crossed syndrome to shoulder impingement is how it affects the movement of your scapula (shoulder blade).

When you reach up towards an overhead position, your scapula should rotate upwards in tandem with your arm. This positions the glenoid (the shoulder “socket”) so that it points more upwardly, allowing your arm to reach greater ranges. However, if this does not occur (whether because of stiffness or muscle imbalances, as we see with the serratus anterior and lower trap weakness / misfiring associated with upper crossed syndrome), your humeral head (the “ball” part of the shoulder joint) ends up squishing tendons and other tissues up into the acromion (that point on the outside of your shoulder), which causes pain and irritation.

Additionally, Upper Crossed aside, if your rotator cuff muscles aren’t effectively stabilizing you shoulder, the humeral head will move excessively upwards on the glenoid (ball rolls up the socket as you move), which can also cause impingement.

Now go see a physical therapist.