Ouch !!! All the boys squirmed at the mention of 'desex-ing'

Back in the day when we all flew four line 'C' kites and they introduced the new fifth line system, a lot of manufacturer's put out retro kits for existing models. It all centred around getting the fifth line right. A 'New' bar that is fifth line ready shouldn't be a problem. The pump leash is the obvious attachment as already stated, but sometimes a 'V' bridle is a better option. Depends on the kite.
As long as you get the length of this line/bridle on the kite right (it basically goes to the centre of the line that would join the two front line wing tip attachments when the kite is fully powered so as not to choke the kite in any way), then the new bar should work fine. Don't forget to put a couple of extra knots in the fifth on the kite for line stretch.
Try the manufacturer; in some cases they released such line lengths...
Oh, and watch out for 'Bow-Ties' !!!
Hope it all works out...
BAP