Custom fields can be populated via the SendReach API, either as part of a new Subscriber record or updating an existing Subscriber record by modifying existing data, or adding new data to an existing record.
The SendReach SDK is available for download at http://setup.sendreach.com/api/api-documentionsdk/. Once downloaded the two folders should de uploaded to your server.
To ADD or UPDATE a Subscriber record go to examples > list_subscribers > 4th example down for ADD a Subscriber and 5th example down for UPDATE an existing Subscriber record (as per first screen shot below).
To add a Custom Field first set the Custom Fields up in Lists > Overview > Custom Fields each of course with a unique tag.
Then back in list_subscribers add the fields as illustrated in the 2nd screen shot below.