If you've finished reading the file, then you could redirect stdin back to the controlling terminal with the command

exec </dev/tty

The script I tried this with looked like:

#!/bin/sh
#This is temp.sh
cat -
# the above "eats" all of stdin
exec </dev/tty
# The above sets stdin back to /dev/tty - the controlling terminal
cat -
# Echo whatever's typed in (Ctrl-D ends the script)

I then invoked it as:

./temp.sh <temp.sh

This printed each line of the script and then started echoing whatever I type in.