In effect you want to do all the exams twice? There is nothing stopping you, go ahead.
I actually did both CPL and ATPL writtens in the pre JAA days due to circumstances. I wouldn't recommend it, except that of course it was a tad easier to pass all the slighter tougher ATPL exams because of my previous experience.
But it's strictly for masochists. Just do the ATPLs unless you enjoy exams otherwise it's a waste.
In any case you need an ATPL to command an airliner. Do you really think you will be in the mood to start studying all over again when you have 1500 hours and working at an airline? Worse still any employer is going to wonder why you didn't do the ATPL exams.