This is easy.
First you'll have to make sure you turn off auto-rule generation (if you haven't already), because it'll use all available ports, and you can't edit the auto-generated rule.
Then create a rule that allows outbound NAT, and in the "Port or Range" box (with the description 'Enter the external source Port or Range used for remapping the original source port on connections matching the rule.') put in 6000:7000