Can I take my iMac to an Apple store and have the Geniuses replace its Fusion drive with a pure SSD?
Yes and no.
If it's under warranty, then it's a definite no, even if you're willing to pay for it.
If it's not under warranty, the answer gets a little more complicated because it depends on the exact model of iMac, its status (i.e. is it considered by Apple to be vintage or obsolete), and to some degree to your location.
The best answer we can give here is that you need to contact your nearest Apple Store and ask them.
My advice would be that if it is under warranty, to just leave things as they are. If it's a real issue for you, then you may have other options (such as using an external SSD as your boot drive, etc).
M-M prohibits manufacturers from requiring that repairs or maintenance be done by authorized service centers using OEM parts and supplies in order for the product's warranty to remain in effect. While customer-initiated product changes unrelated to a failure don't invalidate warranty coverage on other parts of a covered mechanism, M-M does not require manufacturers to upgrade or change their products at customer request, even if the customer is willing to pay for the work.
The "no" that the OP will hear is because Apple chooses not to modify its products still under warranty. This is a rational position. The OP must either wait until the warranty is over (in which case an Apple service center or store might do the work), or take the Mac to to a non-Apple provider, or do the work on his own.