For the actual conversion, use python and CiscoConfParse (GitHub - mpenning/ciscoconfparse: Parse, Audit, Query, Build, and Modify Cisco IOS-style configurations ) to parse the relevant data from the ASA. Once parsed, you can then convert the parsed data into the relevant XML to be configured in NSX via the API.
A word of warning though. If you just blindly migrate the ASA rules, you MUST also set the relevant Applied To for each rule. This becomes somewhat difficult, as you will need to map the IPs used in the rules to match against the relevant VM objects which then can be used in the Applied To (directly or via Security Group membership) of the rule to scope the rule correctly. Failure to leverage the Applied To field will potentially mean that you could cause performance or memory issues due to an un-optimized ruleset.
I found RestAPI tool to migration rules in bulk..... we can add /modify / delete rules from NSX firewall.