How can I get into the subway without killing NSF?

There is a metal hatch on the ground just outside the subway entrance, on the left side. This hatch leads to a network of ducts that you can use to get onto the subway platform while bypassing the main entrance. Avoiding the attention of the NSF personnel on the subway platform will still be something you have to deal with, however.

You can see the location of the hatch at 7:56 in this video:

As said by cloudymusic there is a steam vent that you can access to enter the subway. There is a website that contains a detailed walkthrough of the whole game which shows this -

Explore the shanty town. Inside (1) is a chest with a Lockpick, a Multitool and a Candy Bar. Inside (2) is a chest with a Medkit, a Prod Charger, and a flare. Inside (3) you'll find a Lockpick in a corner. Inside (4) is a steam vent that we will use to enter the subway. The steam vents can also be accessed by opening the indicated panel (inset).

I will say on this one if you're ghosting this mission it's very hard to complete the mission. The game lets you talk to the hostages and tell them to get into the subway, but it takes enough time and noise that the NSF notice and the hostages start doing their "cringing" thing. Usually I've had to try multiple times to complete the mission without touching the NSF or having the explosives go off (they can set them off too).

Of course, it's real easy to just sneak past and fail the mission and you could do that, but the easiest way is to use the tranq darts from the platform you took the screen shot of and then sneak around using that vent to clean up the rest using the prod.

The dialogue does say something to the effect of "several lives" for a few minutes, so they probably intended you to break a few eggs so to speak. But what I always liked about Deus Ex is that nothing is impossible though it might be harder than you-know-what to get it done.