Mix and match templates into a a single document

Our agency enters into 100+ contracts a year with a variety of vendors and uses Adobe Sign for this process.  There is a standard base contract that gets used across the board for all vendors, but there are also a handful of other contract amendments that may be needed based on the services being purchased.  All of these documents require a signature or other info to be entered.   

I know how to setup templates for the base contract and each amendment, but I'm not finding a way to assemble multiple templates into a single contract to be sent for signature via Adobe Sign.  I do not want to combine add all the same fields on every single contract, nor do I want to send each vendor multiple documents for signature.  

How do I combine multiple templates with pre-defined signature and data fields into a single document to send for signature?