How can I add a new target to a blendshape node?

Presuming you have a head mesh called head, that already has a blendshape node input called blendShape1, add a new mesh called headShape3 as the next target in the list:

import pymel.core as pm

bsHeadMesh = pm.PyNode("head")
bsNode = pm.PyNode("blendShape1")
newTargMesh = pm.PyNode("headShape3")
weightCount = pm.blendShape(bsNode, query=True, weightCount=True)

pm.blendShape(bsNode, edit=True, target=[bsHeadMesh, weightCount, newTargMesh, 1])

The weightCount is the number of current targets, which also gives you the 0-based index to add the next new target. The trailing 1 is how much influence should be used for the new target. Shoud probably always be 1.

Here's a tool that will add a new blendshape target mesh based on selection:

import pymel.core as pm

def addBsTarget():
    """
    Select two mesh in order: A new target mesh, and the mesh with blendshape to add it to.
    """
    sel = pm.ls(selection=True)
    if not len(sel) == 2:
        pm.displayError("Please select exactly two mesh, the target, and the base head.")
        return
    newTargetMesh = sel[0]
    bsMeshNode = sel[1]
    bsMeshShape = pm.listRelatives(bsMeshNode, shapes=True, type='mesh')[0]

    bsNode = pm.listConnections(bsMeshShape, source=True, destination=True, type='blendShape')
    if not bsNode:
        pm.displayError("Could find no blendShape node on mesh: %s"%sel[1])
        return

    weightCount = pm.blendShape(bsNode, query=True, weightCount=True)
    pm.blendShape(bsNode, edit=True, target=[bsMeshNode, weightCount, newTargetMesh, 1])
    pm.displayInfo("Added new blendshape target: %s"%sel[0])

http://help.autodesk.com/cloudhelp/2016/ENU/Maya-Tech-Docs/CommandsPython/blendShape.html