Is it possible to place an outgoing underground belt first?

No, you cannot place the outgoing belt first. However, you can actually build the belt backwards, then, without holding any items in your cursor, hover over either end of the underground belt and hit the “Rotate” key (R by default). This will actually allow you to reverse the direction of an already-placed underground belt without destroying it and placing it in the opposite direction.

Unfortunately no, it's not possible to place the outgoing belt first.

What you can do instead is build the belt backwards, then remove both underground pieces and add the ongoing one in the proper spot, followed by the outgoing one.

If you try to remove only one underground piece and replace it, the game will match it up with the existing piece (highlighted with arrows when you hold it before placing) and not rotate it at all.