You realize that's a challenge I can't back down from, right? 😬
In the following example, I have an existing manual rule in the NAT rulebase in the policy package MyPolicy.
I am adding a manual NAT rule to this to do source NAT for MyObject to MyObject-Public.
addelement fw_policies ##MyPolicy rule_adtr address_translation_rule
modify fw_policies ##MyPolicy rule_adtr:1:comments "This is really Rule number 2"
addelement fw_policies ##MyPolicy rule_adtr:1:src_adtr network_objects:MyObject
addelement fw_policies ##MyPolicy rule_adtr:1:dst_adtr globals:Any
addelement fw_policies ##MyPolicy rule_adtr:1:services_adtr globals:Any
modify fw_policies ##MyPolicy rule_adtr:1:src_adtr_translated translate_static
modify fw_policies ##MyPolicy rule_adtr:1:src_adtr_translated:'' network_objects:MyObject-public
modify fw_policies ##MyPolicy rule_adtr:1:dst_adtr_translated translate_static
modify fw_policies ##MyPolicy rule_adtr:1:dst_adtr_translated:'' globals:Any
modify fw_policies ##MyPolicy rule_adtr:1:services_adtr_translated service_translate
modify fw_policies ##MyPolicy rule_adtr:1:services_adtr_translated:'' globals:Any
update_all
(Note: It is helpful to refer to $FWDIR/conf/classes.C when you're figuring out how to do things in dbedit).
Note modifying the NAT rulebase with dbedit has the same basic limitations as the regular rulebase.
Namely, it's easy to add rules to the bottom of the rulebase, but not necessarily "somewhere in the middle" as it involves deleting all the rules below and re-adding them.
In short, it's really worth upgrading your management to R80.x as this is much easier, has better documentation, and more importantly, is supported.