We are looking to add a scannable custom field to the picking process using NetSuite's WMS. Can someone point me in the right direction to do this

Customizing the mobile application is documented pretty well, look at the help section SCM Mobile App Customization. The section Customizing Mobile Page Elements is where you will find adding fields and applying the field elements data capturing and apply them to NetSuite records.

Agreed, the process is fairly simple to follow. There are a few cautions, when scanning a field used to search it can have performance impacts