Early 2009 iMac upgrading to Bluetooth 4.0

Register as an Apple Developer for free with your Apple ID, search for Bluetooth Explorer on this page. After you installed the app you can plug in your Bluetooth dongle and open the app. Press CMD + L and pick your Bluetooth dongle from the list. Reboot your Mac and you're good to go..

This worked for me, I'm using a 2010 iMac with a Trust Bluetooth 4.0 dongle (both Mavericks and Yosemite).

Answer to the original question: Yes. The 'IOGEAR Bluetooth 4.0 USB Micro Adapter (GBU521)' with Chipset: Broadcom 20702A3 is working well in my early 2009 iMac, right out of the box after activating it with Bluetooth Explorer as instructed above @thomas. My Magic Trackpad is working well, as before (had to pair it again, as expected). However, so far Handoff is not supported. I keep investigating if handoff can be enabled using a BT dongle to avoid an invasive fix. Note: despite Host Controller Selector warning, in fact the change IS retained after a restart. Activate the BT dongle only once.

My iMac's System Information reports now after the 'upgrade':

Model Identifier: iMac9,1 (early 2009)

Bluetooth Low Energy Supported: Yes

Handoff Supported: No

Instant Hotspot Supported: No

Manufacturer: Broadcom

Transport: USB

Chipset: 20702A3

Firmware Version: v14 c4096