I am in love with a stranger, but he doesn't even know about my existence. But can I still ask Allah to make this person my life partner in the future during Tahajjud prayer? Will He make the impossible possible?
You can seek from Allah (SWT) to make this person your husband if He knows that he is suitable for you. Your emotions are based on your thinking or sentiments or dreaming, but you don't know the reality of that person nor you know the future. Allah knows everything and He gives the believers the best that He knows for them. You seek from Allah to grant you what He knows the best for you, not what you think it good for you. Keep you supplication conditional according to the best that Allah knows for you.
Wassalam.