I have customer IDs in my older database accounting system, I need these IDs in all customer profiles. I can't find this option anywhere in the customer area in the magento admin, how do I add a custom customer ID?
Let me tell you its possible by custom script.. Firstly you need to create custom attribute for customer.. In order to import you need some common things to identify.. (For I.E email, so that you identify yes this custom ID is for this customer) and then script through which It import on Magento.
The community version does not have this built in, however the enterprise version does.
There are ways around it. You could try to make a extension yourself:
You can also get a 3rd party extension like: https://amasty.com/customer-attributes.html
With either option, it is advisable to install in a test environment before moving to production.