Can I bring in to a store my old desktop computer to have a technician transfer to a new mac

I believe an Apple Store will perform that for you, however you should call your local Apple Store and ask. If you are considering buying from a reseller please ask them.

Also, have you had the chance to look over Move your data from a Windows PC to your Mac?

That is one way, but you will need to call ahead.

Another option, is to move your data from the PC to the iMac yourself.

see > https://support.apple.com/en-us/HT204087

As this would be a non-invasive procedure I can’t see why any local computer technician shouldn’t be able to come to your home and do it there.