Hello.
To my understanding that would not be possible unless there is a parameter within the drives to override their feedback loop. If you attempt to connect them as you mention the result would be that from the point of view of the controller the motor would be at still and then would overcompensate it driving it to the maximum speed which in many cases could be dangerous specially if the motor is already mounted on a machine.
I´d suggest you take a good look at the manual of your selected drive or better yet talk to your dealer first telling him your intention. I´m sure they´ll be glad to help.
I´d also suggest checking the price difference between the simple motor and the one with encoder.
Consider also that it might be possible to add encoders to your simple motors than replace them. That might prove easier if you buy double shaft motors. You have to make the numbers to find the best option for you.
Best regards.