Re: 3 Switches & 2 Lights - possible?
This is a schematic drawing of how 3 way and 4 way switches are connected. The colors do not match your wiring but was done for clarity in following the line. There may be intermediate connections in your wiring also not shown.Ground has been omitted for simplicity
Each 3 way has a common and 2 traveler connections, a 4 way has 2 travelers in and 2 travelers out. One 3 way common is connected to power, the other 3 way common is connected as the hot to a light or lights. The 2 travelers on switch 1 are connected to the travelers on switch 2.
To add a 4 way, you break the connection of the travelers to one switch and connect those wires to the 2 traveler in posts on the 4 way. The 2 traveler out terminals are then connected to the wires that go the the 2 traveler screws on the other switch. This break in the travelers can be made any where along the length of the travelers.
In you particular case the easiest way may be to run a 4 cond + ground from the upstairs switch over head and down to the new switch location. Disconnect the two travelers from the upstairs switch, connect 2 of the 4 conds. to the traveler screws and the other 2 wire nut to the 2 wires you had just disconnected. Keep the paired. Then connect the wires from the upstairs switch to one end of the 4 way and the other 2 wires to the other end of the 4 way.
Be sure you live your life, because you are a long time dead.-Scottish Proverb