Uart / Usart: use Rx-pin only
Without studying the spec-sheet for your particular part, it would be unusual if it did allow you that flexibility. Though according to Bence's comment this one might.

However, that does not mean you can not share pins for different functions.

The key here is to dedicate some pin, or a number of pins, to allow the external circuitry, multiplexers, gates etc., to understand how the device is currently using the pins. That sounds like it means tying up a couple more pins, but if you have enough pins you can share, the total number of pins can be reduced.

In the case of the serial lines, you would of course need to time-multiplex it. That is, you would not be able to do serial communication at the same time as you were using the pins for the other functions.