how can you upgrade to newest OS Catalina from iMac mid 2010 High Sierra? Would this be ever possible?

Hello, depends what you use your mac for, but likely yes...

macOS High Sierra - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

14.3GB of available storage to perform upgrade*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

MacBook (Late 2009 or newer)

MacBook Pro (Mid 2010 or newer)

MacBook Air (Late 2010 or newer)

Mac mini (Mid 2010 or newer)

iMac (Late 2009 or newer)

Mac Pro (Mid 2010 or newer)

https://support.apple.com/kb/sp765?locale=en_US

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

No possible, High Sierra is as far as it can go.

Our 2010 iMac is still on El Capitain. Is it worth upgrading to High Sierra, and is it even possible?

Yes, and I would since High Sierra is still supported.

Thanks BDAqua - this is really useful

Great, think I'm going to go for it then!

OK, great, good luck! :)