How does one become a multi-disciplinary researcher? How can someone become a multi-disciplinary researcher?

For example, can one study a multi-disciplinary domain that combines signal processing, artificial intelligence (neural net, machine learning), robotics, instrumentation and control engineering, and embedded systems? Is there a path that could make a candidate a marketable researcher across all these domains?
To answer the general question: You can be a interdisciplinary researcher by getting multiple degrees in different fields, getting a single interdisciplinary degree, or by getting a degree in one field and, over the course of your career, working your way into another. 

Most academic jobs are hired by departments, and the wide majority of departments are single-disiplinary, though interdisciplinary programs are fashionable right now. 

To be a marketable researcher in a given discipline, you would generally need at least a phd in that discipline, possibly a interdisciplinary phd that centrally involved it (marketability depends a lot on the specific interdisciplinary program, but in general, these will be less appealing than an phd in the actual discipline), or, in very rare cases, a a phd in a different discipline but VERY extensive research in that discipline. When it comes to the latter, things are probably more flexible in the humanities than in the sciences, and often only in one direction (ex. there are philosophy phds in rhetoric departments, but not rhetoric phds in philosophy departments). In general, you need to be able to  teach introductory courses in that discipline as well as in your particular area. 

Some universities do have particular interdisciplinary departments or other interdisciplinary lines, but these are in established interdisciplinary subjects—cognitive science, peace studies, asian cultures, environmental studies...

There are also "cluster hires," where universities look to give a researcher appointments in multiple departments for work on topics like diaspora studies and agroecology that are appealing to administrators but too small to support entire departments. Cluster hires are rare and are not something you can plan a career around. 

There are no lines for unspecified "interdisciplinary" researchers. You need to be able to market yourself under a specific discipline or under an established interdisciplinary program. In general, the latter will be harder.