Sure, first do the query in moloch and look at the url. For example lets say you have the expression "port == 53 & protocols == tcp && packets > 40" the url will look something like
https://host:port/?date=168&expression=port+%3D%3D+53+%26+protocols+%3D%3D+tcp+%26%26+packets+%3E+40
To get the sessions in json format add sessions.json to url
https://host:port/sessions.json?date=168&expression=port+%3D%3D+53+%26+protocols+%3D%3D+tcp+%26%26+packets+%3E+40
To get the pcap for them add sessions.pcap to url
https://host:port/sessions.pcap?date=168&expression=port+%3D%3D+53+%26+protocols+%3D%3D+tcp+%26%26+packets+%3E+40
There is no API manual yet because its still changing