What I want to do is to create a FB to read the signal of input point when it is given a TAG NAME. For example, if the tag name is "I_motor_sensor", so "I_motor_sensor" is send to Read_I_Point FB, then the FB will give the current status back. So is there a way to do this?
Yes, you can read the symbol using FB_ReadAdsSymByName block in the DataExchange library. Info here.. It will take more than one scan though, so should be considered "heavy".
If you want to read them quickly, create a dictonary of references to the variables of interest to you in an intialisation routine, and access them via their key (which you will use a string for).
There's a TwinCAT3 dictonary class in my freely available library, Intecre Utilities, available here. You could try to adapt that for TC2.