How do I add a new item or object to an existing object?
When you use the Twig curly brace {}, key/value pair syntax like you've got imagerConfig set to, you're using the Twig hash literal.

In Twig, if you want to add to an array or hash after it has already been created, you can use the merge filter.

Something like this should do what you're looking for:

{% set imagerConfig = { jpegQuality: 80, pngCompressionLevel: 4 } %}

{% set width = { width: 1000 } %}

{% set imagerConfig = imagerConfig|merge(width) %}

{% set largeLogo = craft.imager.transformImage((logo.getUrl('LargeLogo')), imagerConfig) %}