VF04 modify selection screen

Hi,

I'm an ABAP developer and I've been asked to modify the selection screen of VF04 in the following ways:

1. Make Sort Criterion mandatory

2. Only allow users to enter one value for sort criterion (i.e. remove the ranges and multiple list options)

3. Add an athorisation check on sort criterion as sales office (i.e. we are populating sort criterion with sales office and want to check users have authorisation for this sales office)

I have looked at the code and couldn't find a suitable user exit, I don't want to modify the standard code so my options are:

1. Look at customising to see if it's possible - that's where you guys come in - what is the normal route to modify this screen?

2. Copy the selection screen of VF04 to ZVF04 and add my own code in there and then call VF04 in the background.

Any suggestions are welcome.

Thanks,

Gill