You can do this by inserting the same number range in your FI document type linked to LIV and make this range external (see OSS 203522). However as one of the other experts pointed out that in FI number range is at company code level but at client level in MM so gaps may occur. This is a common problem since 4.6 when SAP introduced two separate number ranges in LIV and FI and there are many OSS notes on the subject. Having the same number is part of the Release Notes on 4.6 please check there on the method.
I used note 634744 to solve this
Hope this helps