var customField = nlapiCreateRecord('itemcustomfield');

customField.setFieldValue('fieldtype', 'INTEGER');
customField.setFieldValue('label', 'Friendly name of your field');
customField.setFieldValue('scriptid', 'scriptidofyourfield'); //custitem will be prepended so don't add that
customField.setFieldValue('showinlist', 'T');
customField.setFieldValue('storevalue', 'T');
customField.setFieldValue('appliestoinventory', 'T');
customField.setFieldValue('appliestoitemassembly', 'T');
customField.setFieldValue('appliestokit', 'T');
customField.setFieldValue('globalsearch', 'T');
customField.setFieldValue('staticfieldtype', 'T');
customField.setFieldValue('fldcurrenttype', 'INTEGER');
customField.setFieldValue('accesslevel', '2');
customField.setFieldValue('fldcurrstored', 'T');
/*optional fields to set
customField.setFieldValue('help', 'Help Text');
//to find the subtab open an item and click on the subtab and then right click to inspect.
//the value will be right after the word custom in the id
customField.setFieldValue('subtab', '-13'); //shows up on main tab
*/

nlapiSubmitRecord(customField);