Welcome aboard, yes Fritzing will do this. You just drag in the I/O extender chip, the switches and a 4 pin generic header in to a sketch, connect the parts in breadboard or schematic then complete the routing in pcb view (the connections will appear there from breadboard or schematic.) The connection to the Teensy occurs via the 4 pin generic header on the board. Have a look at this project (you need to download the .fzz file and load that in to Fritzing) as a poor example. Schematic is complete but breadboard and pcb are not but it gives the idea.
https://fritzing.org/projects/arduino-i2c-io-expander-14-digital-io-and-48-analo
The mcp23017 chip is available as a Fritzing part in the forums here:
assuming that is what you want to use as the io expander.
Peter