Adding Additional Fields in Mergo

Mergo does allow other merge fields in addition to the First Name, Last Name, and the E-mail Address Field

In your draft email.
For each additional field, you will need to place the name of the Mergo field Header in merge brackets {{ }} and specify the object under the Header in the Google Sheet

In the Google Sheets screenshot below. 

A Header for Instrument was created, and placed in Merge brackets in the Google Draft message.
Piano was specified for Billy as the object under the Instrument Header in the Google Sheet
Another header for Food was created and placed in Merge brackets in the Google Draft message
Piano was specified for Billy as the object under the Food Header in the Google Sheet

The finished message would appear in screenshot below