Insycle supports all custom Salesforce fields by default.
When you add a new custom field in Salesforce, it will show in Insycle the next time you log in or within the hour automatically. You can also trigger manual sync of fields metadata at any time, see more details here.
If you're still unable to see Salesforce custom fields in Insycle, you may need to check the field level security, see and make sure that the Visible is checked for the user profile (see more in Salesforce forums here and here).
One way to troubleshoot it is by setting "Field-Level Security" on the relevant field, and checking "Visible" to all profiles.