There are add-on walking feet that can be adapted to use on a 31-15, but these will NOT convert the machine into the type of machine with a built-in walking foot that is designed specifically for heavier/thicker sewing. An add-on walking foot is not the same at all- works very differently.
Not all industrials are designed for heavy/thick sewing. The factory that makes silky bed sheets uses industrail machines, and those wouldn't do at all for automotive upholstery work. By the same token, if you tried to sew a silk blouse with an industrial walking foot machine designed for heavy upholstery work, it would just eat the silk for lunch rather than actually sew it up.
The 31-15 is more of a "tailors" industrial machine, and may not be what you need.
Industrial, Commercial Grade, Professional, Industrial Strength & More- Sewing Machine Buying Guide
Article with information about how an ad-on walking foot really works and what it does