Yes it is possible. You can add either PG or VG to your juice to dilute it but note that you will be reducing the flavour strength. You can maintain the PG/VG ratio of your existing juice buy adding the relevant ratio of PG/VG to it.
To decrease the nic from 6mg to 3mg you will need to double the liquid quantity. In other words, if you have 10ml bottle, you will need to add 10ml of PG/VG. A 30ml bottle, add 30ml of PG/VG.
Hope this helps