Your freewheel/hub are designed to have the thru-axle provide support to the 4/5 bearings which then provides the rigidness for the rear triangle. Without the support and with power on the drive-train, you will destroy both very quickly as the bearings will have nothing to rest on (they will collapse where the freewheel joins the hub and quite likely do other damage potentially to rear mech, spokes etc. Dont do it.
A friend waited 2 months to get his hands on the right Tacx axle for his daughters blue-matic - hard to get it seems.