完全可以加上介词 to。其实，你的句子本来应该有 to，只是出于简洁的考虑，这个 to 承前省略了。

I always try to be kind to people around me, even (to) strangers.

但是，如果省略介词会导致误解或意思不清，则不能省略。如：

He can be quite stand-offish and rude, even to his friends.他有时会很冷淡，很粗鲁，甚至对自己的朋友也是如此。

Beth is very competitive, even with her friends. 贝斯非常好胜，就是在朋友中间也是如此。