I have not done anything like that in SW but if I had to, I would save the drawing as PDF and change the infill using Inkscape or Illustrator

As a variation to Jack's solution:

Many PDF editor's can add text highlights. Save the drawing as pdf, open its pdf in a pdf editor and you can select the balloon text to add a text highlight.

As Adobe Acrobat Professional isn't exactly cheap, I'd recommend using PDF-XChange Editor:
https://www.pdf-xchange.com/product/pdf-xchange-editor
This software enables you to do 99% of Adobe's overpriced software, at a fraction of its costs. It even works with a similar interface, shortcut keys, etc. I've been working with Adobe for over a decade, but about five years ago I moved to PDF-Xchange due to cost reductions, and frankly I don't even regret this.

Top tip: you can even use it legally for 'free', with functional limitations. Some features are freely available, other do function too, but will leave an additional watermark to a document. Fair chance that text highlighting does not leave a watermark.

And no, I do not have stocks of this software developer, nor am I otherwise affiliated ;)