We have to square up the new vertices a bit. Deselect (AKEY) all vertices. Select the two vertices (SHIFT+RMB) that are just to the left and right of the center vertex at the top. This is easier to do if you Rotate (MMB) the Front View slightly.

In Top View move (GKEY) the two vertices up until they are even with the center vertex. Don't forget your MMB. Repeat with the other side.

Deselect (AKEY) all vertices. Now we have something like this:

Go back to Front View (NUMPAD 1) and drag a box to select (BKEY) the top 3 vertices on the right side of the model. You will only see 2 of them; the third is directly behind one of the others:

Zoom out (MMB) to make room in the Front View for the crosspiece. With the selected vertices, Extrude (EKEY) them out to the right. Deselect (AKEY) all and repeat on the other side:

Rotate (MMB) the Front View to see your efforts:

Move your cursor to the Side View window and select (BKEY) the entire top row of vertices and Extrude (EKEY) them straight up to make the crosspiece thickness:

Deselect (AKEY) all. This will now make our Front View look like this:

Select (SHIFT+RMB) the two center vertices on either side of the top:

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