Is there any way to programatically update picklist values in Zoho CRM?

Is there any way to programatically update picklist values in Zoho CRM?

This can't be a unique problem. I've created an external API at Amazon and need to be able to update fields within a module based on the information pulled from that API. I am an experienced developer but am not experienced with Zoho CRM.

Here are the steps I need to accomplish:

1) Connect to my external API, sending a zipcode from a module field and receiving a JSON list of objects matching that zip code. I've already accomplished this and can populate a related list based on that data.

2) In the module, the user must select ONE of these returned data objects. So far, I have discovered no way to do this. I had assumed that one can populate a picklist with values from a related list but that doesn't seem to be an option. I know that one can create a lookup field based on a related list but there doesn't seem to be a way to get it to work with an existing related list that was populated by a function.

3) Once the user has selected the object from some kind of list, other field values in the module must be updated to match items in that data object. I've figured out how to update fields within a given record using a custom function so this isn't really a problem as far as I can tell.

The real challenge seems to be presenting the results of an external data call so that the user can choose which data to actually apply to the field values. Any solution would be welcomed.