Can a submit button script append (not attach) a selected document to a fillable form for submission

Unless all of your users have Acrobat and will install a script on their local machine it's not going to work, and even then it will be very tricky.

I would say to outsource a developer. What you're asking involves advanced scripting knowledge and I'm not even sure if it is possible to to do all that from Acrobat and expect it to work (or not) with other software platforms..

You're not asking for one script, this is a project that involves many scripts.

You may be even asking for a custom made plug-in (paid-for solution of course).

You also need to consider all other users out there  who will open this PDF in a computer and run which ever version of Acrobat they have (or web browser like you said, or a third-party PDF viewer) in different operating systems.

This pre-requisites alone  will have an impact on where the script intends to save a file or where to call it from, which in the context of trusted privileged functions it makes any script almost impossible to handle not mention knowing when to save a file with Reader enabled rights.

If you're expecting the script to know where to save the converted Word to PDffile and how to call for it, you got yourself in deep waters. It seems that you have a very complicated project on hand.

I'd suggest that you read these links below carefully (which are all excellent JavaScript scripting turtorials by Adobe Community Professional Thom Parker) :

• File Paths in Acrobat JavaScript 
• How to Save a PDF with Acrobat JavaScript 
• Folder Level Scripts 
• Trust and Privilege in Acrobat Scripts 
• Using Trusted Functions 

These topics just cover the very  basics of some of the things you've asked to develop such project.

If after you read those articles you feel that you know how to tackle this, then it is easier for the developers in this community to take you by the hand and show you the way.

But if you feel that you don't understand anything in those articles I would seriously advice to hire a professional.

As for myself, I've been  learning JavaScript  for  a little more than a year now (following some of the developers in this community) and I wouldn't even  know where to begin with  your project. 

To be honest you're better off encouraging your users to get a paid subscription of Acrobat Pro DC (full desktop editor) which already has all the built-in tools to create batch actions, customizable action wizard, and more.  Or, if money is an issue at this time,  also consider encouraging your users to subscribe to Adobe Acrobat Export plan which is $1.99 a month.

This gives your users to have the flexibility to this online from a web browser or even a tablet or mobile device.

See more here: https://acrobat.adobe.com/us/en/acrobat/export-pdf-online-pricing.html

Also see how it works here: https://helpx.adobe.com/document-cloud/help/using-exportpdf.html